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The navigability capacity under the natural development of the Meta River was evaluated to identify the hydrologic, hydrodynamic, sedimentologic, and morphologic conditions to propose hydraulic works for establishing a navigation channel. The Meta River is one of the main rivers in Colombia with a high economical potential through its fluvial navigation. A 2D numerical modeling was conducted by using MIKE-21C for a representative sector of 75 km with braided and sinuous segments. The 2D model was calibrated by using water level and flow rate time series data, sediment data, and field data of velocity with ADCP, bed sediment sampling, and bed morphological changes with bathymetries performed in August 2012 and February 2013. The evaluation also included hydrological analysis; the morphology of banks, islands, and sandbars; and the analysis of satellite images from 2000 to 2012. Results of depth, velocity, shear stress, and spatial and temporal bed level changes were obtained and analyzed. The results showed that the natural development of the river allows navigation eight months per year on average (between May and December) without need of hydraulic works. However, between January and April, the depths are not enough for navigation. The river has a high morphological dynamic linked to the wide flow rate variation that influences the hydrodynamic and sedimentologic conditions. This dynamic causes important short-term morphological changes on the thalweg's alignment, changes in the flow rate distribution percentage on river branches, and other morphological changes that modify the active river channel. These results suggested that, based on the natural river behavior, the hydraulic works oriented to river training, which progressively establish the alignment and depth of a navigation channel, are more appropriate for the Meta River than fixed and hard solutions.

Extensive studies have been completed on stream profile and culvert interaction, particularly addressing downstream scour and upstream aggradation processes. However, geomorphic placement of culverts has not been studied in comparable detail. To meet dual objectives of aquatic organism passage and stream profile maintenance, a roadway culvert was constructed below the thalweg profile in a manner similar to a "stream simulation" culvert. The installation included an inlet headwall to maintain stream grade and a scour pool for downstream energy management, as culvert outlets are typically marked by large downstream scour pools associated with expansion of the exiting flow. This countersunk culvert was physically modeled at model and prototype scales to determine effects on scour pool development and downstream energy management as functions of countersinking and longitudinal pipe slope. These physical methods indicated that energy dissipated within a countersunk culvert section may reduce the required length, width, and depth of scour pools from previously reported values. The extent of scour with increasing culvert countersinking generally follows a similar pattern for all geometric measures—a reduction in the amount of scour compared with that of a lesser countersunk culvert. This is particularly true for the percent countersunk values greater than 20%. It is likely that the presence of the downstream knickpoint (forced by proper placement of a riffle crest) and the associated backwater pool provides increasing resistance to the turbulent jet that typically exits from a culvert, thereby forcing greater turbulence and greater energy dissipation within the culvert.

Ten years of river restoration at the Pueblo of Santa Ana have included projects focused on bioengineering, native plantings, and habitat creation for the federally listed endangered Rio Grande silvery minnow (Hybognathus amarus). When a natural river meander threatened to erode bankline adjacent to ageing cottonwoods and flank an instream grade control structure in the Rio Grande, the Pueblo and Bureau of Reclamation collaborated to create an ecologically based solution. A modified bendway weir project was implemented which included a series of 13 earthen-buried weirs. Earthen material was used to reestablish 2,000 linear feet of bankline from a 10 feet vertical height, 50 feet in length, to meet the water’s edge. During construction, a large scour pool associated with the vertical bank was filled. Additionally, the channel was moved toward the opposite bank and a spoil pile berm was removed that increased floodplain overbank inundation. Both scour pools and floodplain overbank inundation are important habitats during the silvery minnow’s life history leading the Pueblo to ask “how did reestablishing the bankline change the river channel at the project site and what were the short-term effects on river habitat and the fish community? Localized geomorphic and hydrologic changes were identified using channel cross-section surveys before (2008), immediately following (2009), and after construction (2010, 2011). The Pueblo used relative comparisons of the project site to reference sites for habitat change analyses based on ecological criteria described for the silvery minnow. Channel characteristics (thalweg location, habitat type, flow, depth, and sediment type) were evaluated for short-duration effects from project implementation. Fish community survey data was collected three times per year (March, July, November), before and after construction. Minnow catch per unit effort, fish species richness and diversity were analyzed using non-parametric statistics to identify differences. Lastly, the five most abundant fish species were compared for dominance before and after construction. The Pueblo’s analyses indicated that the project produced localized changes to the river channel resulting in a two year shift of the river’s thalweg. This resulted in some generally positive changes related to the silvery minnow’s preferred habitat and overall fish community. Despite these changes, the silvery minnow population was relatively unaffected leading the Pueblo to determine that the project successfully met the bankline reengineering objectives while minimizing disturbance to the species. 2637 World Environmental and Water Resources Congress 2012: Crossing Boundaries © ASCE 2012

Dam removal is crucial for reconnecting river habitats, restoring passage of fish and other aquatic organisms, and restoring the free flow of water and sediment. However, removal of obsolete dams is often resisted due to concerns of releasing sediment and initiating channel instability. Two dams on the Musconetcong River in northern New Jersey have been removed by Princeton Hydro LLC in 2008 and 2009 as part of a watershed-wide effort to remove or breach all major obstructions to restore the river to its original free-flowing state. The two dams were consecutively situated 0.6 miles apart and their removals provided lessons in design and construction as well as offered an opportunity to study the geomorphic response in the form of bed elevation changes and sediment size through pre- and post-removal monitoring. The Musconetcong River, within the Delaware River watershed, is 89 km long and has a watershed of approximately 400 km 2 . The watershed has mostly rural (forest and agriculture) land use with a few small towns, the largest being Hackettstown (population ∼10,000). The Musconetcong flows southwest and is predominantly an alluvial stream with mostly shale and carbonate bedrock lithologies (Drake et al., 1996). The importance of measuring, predicting, and mitigating the effects of these dam removals is augmented by 24 miles of the Musconetcong River being federally designated as National Wild and Scenic in 2006 (Musconetcong Advisory Committee, 2003). For the removal of the downstream dam, the permitted engineering design called for the installation of boulder rock vanes that would maintain a low, in-stream, grade-control to stabilize some of the sediment, allow fish passage, direct flow away from downstream bridge abutments, and blend in with the character of the waterway. Unforeseen conditions that arose during construction necessitated in-field adjustments, or adaptive management. The dam structure itself was notched weeks prior to the start of construction to allow for partial dewatering of the impoundment. Due to downcutting and channel formation, this process drastically altered upstream bed elevations, such that the proposed design was no longer appropriate. Through collaboration with NJDEP Dam Safety and other project partners, the in-field redesign of not only the elevations, but the configuration and layout of the weir, was crucial to moving the project forward. Ultimately, two boulder rock vanes were installed to maintain an in-channel grade control to limit channel incision and retain some of the impounded sediments, while allowing for fish passage and directing flow away from abutments of a downstream bridge. In the course of construction, sediment was managed in several ways. First, some impounded sediments were permitted to migrate downstream after notching the dam to dewater the impoundment. Second, loose fine sediments (dominated by silt and sand) were excavated from the channel center and placed on the channel margins during the installation of the boulder rock vanes. Sediments were excavated to the approximate depth of the original stream bottom where highly resistant, consolidated layers were exposed to serve as a solid foundation for the footer stones of the boulder rock vane. These design details are important in understanding initial local patterns of erosion and deposition and may be important in interpreting the long-term geomorphic response of the reach. The upstream dam was crudely constructed of concrete poured over a base of river cobbles most likely gathered from the impounded reach. The impoundment was created to provide a swimming area and beach, which was periodically enhanced with imported sand. The structure had two 36-inch (approx) concrete culverts on the right and left sides of the channel that were set nearly flush to the riverbed. As a result, the impoundment was limited in extent, contained entirely within the river channel and sediments had only accumulated in the center of the channel. Over-topping flows had caused a bank failure on river right of the structure. Concerns of project partners and regulators varied widely regarding the fate of impounded sediments, the risk of channel instability and the need for in-stream grade control structures. After much discussion, the final design avoided unnecessary and costly in-stream structures and allowed for gradual geomorphic adjustment. Permits required the rebuilding of the eroded bank and placement of cobbles and boulders in the channel. To minimize the use of imported materials, the bank was built-out with cobbles from the dam and impounded sediments immediately upstream of the dam. Initial geomorphic surveys of the riverbed in the vicinity of and between the two dams have shown areas of erosion and deposition. These surveys have established a set of control points along the river channel between the two dams, and confirm the downstream movement of a sediment plume and localized areas of erosion. At the upstream dam, comparisons pre- and post-dam removal surveys show greater than 100 cubic meters of sediment being both eroded and deposited within the site. Most but not all of the erosion occurred around the newly exposed sediment bar upstream of the former dam, where the thalweg has reestablished itself following the dam's removal. Areas that were excavated during removal have experienced deposition. Most of the deposition occurred downstream and on the left-hand bank. Due to the two low flow culverts in the former dam, a mid-channel sediment bar remains. At the downstream dam site, erosion has removed up to 1.1 m of sediment from the bed in places while depositing up to 0.5 m sediment in others. As sediment from the former impoundment migrated through the project site, areas excavated during the removal became areas of deposition following the removal, and; alternately, areas in the channel margins where sediments were placed experienced gradual erosion. Grain size analysis shows a coarsening of the riverbed over the first nine months since removal. Grain size analyses were done upstream and downstream of the dam sites as well as at two locations between the sites. Wolman pebble counts (Wolman, 1954) were completed using the random walk method at each of the six sites. The largest change in grain sizes at the four sites occurred upstream of the downstream dam site, where there was a significant coarsening of the sediment from October 2008 to June 2009. This has most likely occurred from the increase in energy upstream of the dam post-removal, which has transported many of the fine-grained sediments downstream. Downstream of this dam site sediment size has not significantly changed, suggesting that the fine sediments have been transported downstream far enough to leave the site. Surveys of the channel thalweg above and below both dams also show a pulse of sediment migrating slowing from the uppermost impoundment areas. Long-term monitoring of the channel thalweg may reveal reach-level changes in channel slope.

In addition to the original design and construction information from the 2003 EWRI publication, this paper also includes follow-up regarding the Kansas River bank stabilization project condition. The Kansas River is a major Missouri River tributary. The project site is 43 miles (70 kilometers) upstream from their confluence, with a drainage area of 58,610 square miles (151,800 square kilometers). The channel ranges from 590 to 1180 feet (180 to 360 meters) wide in an 8860-foot (2,700-meter) floodplain. The project area was a channel bulge along the left bank of the river, at an inflection point where the channel meanders from the north (left) side of the floodplain to the south side of the floodplain. A bridge is located downstream, in the middle of an east-west reach; downstream of the bridge, another meander returns the channel to the north. The sandy channel banks ranged from 20 to 30 feet (6 to 9 meters) tall, stood at slopes from 1.0 horizontal to 1.0 vertical, to nearly vertical, and tended to collapse into the channel thalweg and washed away whenever water levels receded.

Scour analyses are required for stream projects such as stream restoration, utility crossings, bridges, streambank protection, levees and floodwalls. In lieu of sophisticated numerical models, simplified approaches are often utilized, especially for reconnaissance and feasibility studies. Neglecting local scour such as scour at bridges, the total scour using these simplified techniques is comprised of long term, general, bend, dune formation, and thalweg formation. There are several methods within each of these scour components and calculations are often tedious and repetitive. This presentation provides information on a free spreadsheet developed by PBS&J that computes the most commonly use scour methods. The power of the spreadsheet is in the use of inputting the variables once and computing the results using numerous methods. These scour methods are

The shoaling rate in Blind Pass entrance channel approximately equals the net southward longshore sediment transport rate. Most of the shoaling occurs along the northern side of the channel. Flow measurements and numerical modeling reveal that the ebb current is approximately twice as strong in the thalweg along the south side as compared to the rest of the channel. In contrast, the flood current is mainly uniform across the inlet and is stronger than the ebb flow over the northern portion. This flow-velocity distribution induced by a nearly 90-deg bend of the channel contributes to the shoaling pattern. Two years after the 2000 dredging, the shoal along the north side became shallow enough to induce wave breaking. Distinctive shoaling patterns influenced by seasonal wave climate were observed. Sediment deposition in the channel is closely related to the passages of cold fronts, which significantly increases the sediment supply by longshore transport.

The processes controlling fine sediment deposition, storage, and transport within laterally constricted pools of the North Fork Cache la Poudre River, northern CO, were investigated following the 1996 release of approximately 7,000 m 3 of sediment. The release provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- and two-dimensional sediment transport and hydraulic models against the field observations. Sediment transport within the North Fork following the release is similar, on a smaller scale, to transport of sediment following dam removal. Sediment from the release filled pools up to 3.5 m deep, and defined the starting condition for the modeling. During an experimental release of water from the reservoir in March 1997, sediment was scoured from the thalweg of pools and transported by a combination of translation and dispersion. Some sediment became deposited as higher lateral channel bars following thalweg scouring as sediment moved from pool to pool downstream during subsequent flow releases. At high discharges, the development of a strong shear zone prevented degradation of sediment deposits within the eddy. One-dimensional HEC-6 results provided the greatest agreement between predicted and measured pool bed elevation change. Results from the two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics, and provided input for bed load and total load computations which accurately delineated areas of scour and deposition. Less energetic fluvial systems could result in longer-term deposition of sediment than was experienced on the North Fork.

Instream gravel mining can alter physical processes of streams and can affect aquatic habitat. The removal of alluvial material from a streambed has direct impacts on the stream's physical boundaries, on the ability of the stream to transport and process sediment, and numerous associated habitat qualities. Local physical effects can occur immediately following sediment removal, including: (1) changes in channel geometry, (2) decreased bed and/or bar elevation, (3) changes in bed and/or bar substrate composition, (4) reduced form roughness, (5) loss of instream roughness elements, (6) decreased average stream depths, and (7) changes in velocity patterns. In addition, increased turbidity, changes in sediment transport patterns and timing, and changes in air and water temperature, especially if riparian vegetation is removed, may also occur. All of these effects can diminish normal stream habitat function. The effects of floodplain pit excavation can include: (1) channel capture, (2) bedload trapping and channel incision, (3) reduced water quality in the stream, and (4) altered water tables. To counteract these negative effects, a gravel mining reclamation plan must address geomorphic functions and habitats. This can be accomplished in several ways including: (1) retaining the wet edges of bars, (2) mining from downstream toward the interior of bars, (3) mining above ordinary high water, (4) allowing a significant portion of the bedload to pass through the mining site, (5) retaining mature riparian vegetation, and (6) maintaining or increasing topographic and thus hydraulic complexity. Minimizing the deleterious effects of instream sediment removal would include not disturbing bar form in its natural configuration and size, for elevations related to a design discharge. The selection of an appropriate design discharge should be based on site specific retention of the physical processes that either create or naturally modify specific fish habitat elements, such as (1) bed mobilization and redd scouring, (2) pool scouring and maintenance, (3) flushing flows for spawning beds, or (4) preventing fine sediment introduction from removal surfaces (or from modified hydraulics) during egg incubation. The negative effects of pits can be addressed by (1) not mining deeper than the thalweg depth, (2) creating simulated geomorphic features such as sloughs or oxbow lakes, (3) maintaining or improving riparian vegetation between pits and stream banks, and (4) providing connections to off-channel habitats suitable for target species.

The project, located in Plaquemines Parish, Louisiana on the west bank of the Mississippi River at Mile 4.7 Above Head of Passes, diverts water and sediments into West Bay through a dredged conveyance channel. The objective of the project is to restore 10,000 acres of vegetated wetlands in shallow water within the Mississippi River delta over the next 20-years. The sediment diversion channel is being constructed in two phases: 1) Construction of an interim channel to accommodate a 20,000 cfs discharge at the 50% duration stage of the Mississippi River (completed November 2003), and 2) Modification of the channel to accommodate full-scale diversion of 50,000 cfs at the 50% duration stage (scheduled September 2005). The enlargement phase will be built upon completion of a period of intensive monitoring of project performance. Prior to construction of the diversion channel, a large natural gas pipeline located in the outfall channel pathway was lowered for safety reasons (completed May 2003). Contingency plans for closing the diversion channel would be implemented if hydrographic monitoring of the Mississippi River navigation channel indicates the Thalweg of the river is migrating toward the diversion channel or if river shoaling substantially increases downstream of the diversion. The project is the largest freshwater and sediment diversion ever constructed for environmental restoration purposes. Lessons learned from this project will be applied to several large-scale alternatives under development as part of a comprehensive plan to restore and sustain coastal wetland habitats in Louisiana.

The Kansas River is a major Missouri River tributary. The project site is 70 kilometers upstream from their confluence, with a drainage area of 151,800 square kilometers. The channel ranges from 180 to 360 meters wide in a 2,700-meter floodplain. The project area is a channel bulge along the left bank of the river, at an inflection point where the channel meanders from the north (left) side of the floodplain to the south side of the floodplain. A bridge is located downstream, in the middle of an east-west reach; downstream of the bridge, another meander returns the channel to the north. The sandy channel banks range from 6 to 9 meters tall, stand at slopes from 1.0 horizontal to 1.0 vertical, to nearly vertical, and tend to collapse into the channel thalweg and wash away whenever water levels recede. During the wet decade of the 1940s the channel moved 200 meters eastward. The great 1951 flood resulted in an eastward shift of over 300 meters, plus a significant channel shift southward, upstream of the bridge. At the channel bend between the project site and the bridge, three long right bank rock dike structures were built by Douglas and Leavenworth Counties in the mid 1950s; three short left bank structures were added in the late 1950s. These structures have been effective in stabilizing the reach upstream of the bridge. In the late 1970s, a Corps of Engineers demonstration project stabilized the bend upstream of the project site, mainly using windrow revetments that eventually launched and formed relatively uniform and stable bank armor. The 1993 flood initiated a left bank channel bulge between these two stabilized bends. The bulge steadily pushed eastward, progressing about 250 meters by 1999 and threatening existing stabilization. Many different stabilization methods were considered, including Bendway Weirs, Bank Revetments, Rock Dikes, Longitudinal Peak Stone Toe Protection, and L-Head Dikes. The L-Head dikes were selected as a combination of the most cost effective and environmentally friendly alternative. The first three dikes were constructed in early 2002, and the fourth is scheduled to be installed in early 2003. The structures have already successfully moved the channel thalweg away from the bank, allowed the channel slopes to vegetate, and have begun a process of accretion between the L-Heads and the banks.

Stantec was hired by Duke Energy to complete a feasibility study for restoring the Tuckasegee River to more natural conditions following the proposed removal of the Dillsboro Dam. Two specific questions regarding restoration of the river following dam removal were asked: How will the river look? and How will the river sound? The first question was answered by using an innovative approach involving digital photo renderings. The digital image showed what the Tuckasegee River would look like after the removal of Dillsboro Dam. Stantec used a large number of digital images taken at various locations along the river. The digital photo of the actual dam was altered to show how the site might look before and after the restoration of the river. The question of how it would sound was answered using a standard traffic sound meter. Sound readings were taken at various locations along the river. Duke Energy made releases from upstream dams to allow for sound measurement under both high and low flow conditions. With the readings, Stantec was able to determine the sound would be similar before and after dam removal the restoration of the river. In addition to answering these questions, Stantec developed a conceptual design based on a Rosgen Level III assessment of the Tuckasegee River. The assessment evaluated the river's cross sections, longitudinal profiles of the water surface and channel thalweg, and characterization of the bed material above and below the dam. Stantec developed a conceptual design for the restoration that included the following: creation of a W weir at the location of the dam; narrowing of the channel; and stabilization of the new banks. This design was used to answer the specific questions raised by local stakeholders regarding the proposed dam removal. Duke Energy has yet to make a decision whether to proceed with the removal of the Dillsboro Dam and restoration of the Tuckasegee River. The decision will depend on the results of negotiations related to the relicensing of hydroelectric projects on the Nantahala system. If project proceeds, it will be only the third dam removal project carried out in the State of North Carolina.

Proposed placement of dredged material in a deep trough of the Potomac River below Alexandria, Virginia led to environmental concerns that the deposited material would disperse to nearby shallow bottom habitats. Between December 15, 1999 and February 5, 2000, approximately 328,000 cubic yards of dredged material, representing 212 split- hull barge loads, were placed in the trough. Monitoring efforts included pre-, during, and post-disposal deployment of bottom - mounted optical backscatter sensors (OBS), and two sediment profiling imagery (SPI) surveys, one four months following placement and a second a year later. Examination of the OBS data indicated that pulses of extremely turbid (>1,000 NTU) water moved through the study area prior to the onset of disposal operations, and intermittently throughout the study period. Bottom waters in the channel thalweg were generally much more turbid than bottom waters over adjacent shoals. These high background turbidities tended to mask any elevated turbidities linked to the disposal events. The few turbidity pulses that were discerned to be associated with disposal events were relatively small (< 250 NTU) and ephemeral (< 1 hr). SPI images collected four months after cessation of disposal and a year later revealed "signatures" of sandy dredged material within the trough. No evidence of significant dispersion of dredged material outside of the trough was seen. Following cessation of disposal, benthic assemblages in the vicinity of the site appeared to be undisturbed. Although long-term data describing bathymetric changes within the trough are needed to evaluate the stability of the deposited material, no significant long-term (1.5 year) movement of materials out of the trough was observed.

Presented herein is a framework for evaluating the capacity of very small hydraulics models, including micro-models, to aid the design of channel-control works involving structures such as dikes. The framework comprises considerations of similitude, relaxations, scale effects, compensating effects, and the necessary experiments to determine the limits for effective model use. The framework suggests that small models can be useful as design aids in situations where a channel thalweg has only one-degree of translation freedom for movement; i.e., the thalweg is constrained to vertical movement in a long constriction (constriction length exceeds at least one channel width) formed in uniform sediment. However, model utility diminishes quickly in situations where alluvial channels have a greater degree of translation freedom; notably, when they can move laterally or armor.

Techniques for the hydraulic restoration of placer-mined streams and floodplains were developed in Denali National Park and Preserve, Alaska. The two-year study at Glen Creek focused on a design of stream and floodplain geometry using hydraulic capacity and shear stress equations. Slope and sinuosity values were based on regional relationships. Design requirements included a channel capacity for a bankfull discharge and a floodplain capacity for a 1.5- to 100-year discharge. Several bio-engineering techniques using alder and willow, including anchored brush bars, streambank hedge layering, seedlings, and cuttings, were tested to dissipate floodwater energy and encourage sediment deposition until natural revegetation stabilized the new floodplains. Permanently monumented cross-sections installed throughout the project site were surveyed every one to three years. Nine years after the project began, a summer flood caused substantial damage to the channel form, including a change in width/depth ratio, slope, and thalweg location. Many of the alder brush bars were heavily damaged or destroyed, resulting in significant bank erosion. This paper reviews the original hydraulic design process, and describes changes to the channel and floodplain geometry over time, based on nine years of cross-section

The National Park Service (NPS) is required to assess risks of flooding to existing and proposed structures within the boundaries of the NPS system. Within Denali National Park and Preserve (DNPP and 2) There is a moderate to high potential for bank erosion. The development of a return period versus flood depth/bankfull depth curve is utilized to delineate inundated areas for a specific flood event. The bankfull depth is taken as the distance from the top elevation of mid-channel gravel bars to the thalweg. Flood depths for varying return periods are calculated using HEC-RAS and cross-sections from the Teklanika and Toklat Rivers. By determining the bankfull depth on rivers with similar hydrologic and hydraulic characteristics as the Teklanika and Toklat Rivers, the curve serves as a tool for calculating flood elevations and corresponding floodplains for specific flood events. A two-dimensional risk analysis based on potential bank erosion and consequence severity is used to assess localized bank erosion resulting from flooding. Bank characteristics and fluvial geomorphic variables, such as the bank height/bankfull depth ratio and channel width/depth ratio, are used to assess potential bank erosion. Potential loss of existing and/or proposed structures in a flood hazard zone is categorized with low, moderate, or high consequence severity. After NPS review, the method may be utilized on similar reaches.

This paper presents results from a three-year field study that is being conducted to determine the morphology, processes and migration rates of nine knickpoints in the cohesive clay beds of the Yalobusha River System, Mississippi. The roles of surface erosion by hydraulic shear stress and mass failure during the upstream migration of knickpoints are evaluated. Results of submerged jet tests reveal a wide variation in the erosion resistance of the streambeds. Values of critical shear stress, τc, span almost four orders of magnitude with a median of 31 Pa, while values of the erodibility coefficient, k, span about three orders of magnitude with a median of 0.022 cm 3 /N-s. To relate τc and k-values to the erosion potential of flows, an excess shear stress approach is used to estimate erosion rates using values of average boundary shear stress, τ0. Most flows are competent to erode streambeds composed of the Naheola formation (τc = 2.24 Pa), while only the deepest flows and steepest hydraulic gradients generate boundary-shear stresses great enough to erode the Porters Creek Clay formation (τc = 199 Pa). Repeated thalweg surveys indicate that knickpoints formed in Naheola clay have migrated at an average rate of 7.2 meters per year since 1997, while knickpoints in Porters Creek Clay have migrated at an average rate of around 1.2 meters per year over the same period. Comparison of calculated erosion rates and observed knickpoint retreat rates indicates a discrepancy between migration rate and available hydraulic shear stress. Additional mechanisms have hence been identified as contributing to the migration of cohesive knickpoints: a cyclical mass-failure mechanism, and detachment of aggregates by upward directed pore-water pressure on the falling limb of hydrographs. To assess the role of mass failure in knickpoint retreat a combination of finite-element hydrologic and limit equilibrium slope-stability modeling was carried out. Thalweg survey data were used to construct a series of finite-element meshes based on the geometries of the knickpoints during migration, while head and tail water elevations were taken from stage data logged every 30 minutes above and below the knickpoint. At one example location in Naheola clay, Big Creek, every observed event greater than 0.3 m has been extracted from the stage record and simulated. Where a mass failure was indicated by a factor of safety close to or below one, the predicted failed distance was compared with the observed knickpoint retreat distance at the next survey. Results at this location show a close agreement between modeled and observed retreat rates. Quantification of knickpoint migration rates and processes in the field enables river managers to accurately determine the locus of incision at a particular time, allowing estimates of sediment sources and yields. This study has also shown that knickpoints formed in resistant substrata may act as a form of natural grade control, while knickpoints formed in more erodible substrata may require direct intervention by management agencies.

Reclamation proposes to preserve archaeological site 24BE529 from erosion by the Red Rock River in southwestern Montana. The Red Rock river is naturally meandering towards site 24BE529, slowly eroding the bank containing the resource. Federal historic preservation laws require Reclamation to address the impact. The preservation plan calls for the installation of three or four Bendway Weirs, also called Thalweg Displacing Barbs. The order of construction excludes all equipment activity in the stream. The barb materials will be large stones, up to 24 inches (61 cm) in diameter, quarried nearby. The bank buttressing material will be smaller stones, less than 12 inches (30 cm) in diameter. A top soil will be placed on both the finished barbs and the bank buttressing material. The soil will be seeded with native grasses and willows, forming a natural looking bank.