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This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub-centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi-annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre-scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ13C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.

Previous estimations of sediment flux for the Salinas River of central California were based on data collected in the 1970s and assumptions of time invariant suspended sediment-discharge behavior. The goals of this study were to estimate sediment flux from the Salinas River using data from 1967-2011 by incorporating time dependent behavior and reassess the role of El Niño Southern Oscillation patterns in inter-decadal sediment load. This study builds on previous findings that time-dependent suspended sediment behavior in this system is controlled in part by antecedent hydrologic conditions. The condition of temporal dependence was further tested herein through comparison of flux estimates obtained using time-dependent formulations and a multivariate approach incorporating hydrologic factors. Longer sampling records and incorporation of decadal scale behavior or antecedent hydrologic conditions resulted in average annual load estimates of 2.0-2.9. Mt/yr with 95% confidence intervals of ±25 to 202%, in comparison to earlier estimates of ~3.3. Mt/yr. Previous overestimation of sediment load is due largely to the extrapolation of suspended sediment behavior from a decade of high sediment concentrations to the entire record, and the use of log-linear regression techniques on a non-linear system. The use of LOESS methods lowered QSS estimates and decreased confidence interval size. The inclusion of time-stratified and antecedent flow indices further decreased QSS estimates, but increased confidence interval size. However, temporal dependence of the CSS- Q relationship violates the assumptions of single base period regression, which suggests that time-stratified rating curves provide more realistic estimates of sediment flux means and uncertainty. The majority of suspended sediment was transported by flows of ~25-90 times mean discharge depending on transport constituent (fines or sand) and estimation method. Periods of differential suspended sediment behavior changed the relative importance of rare floods due to changes in the relationship of suspended sediment concentration vs. discharge. El Niño years dominated the sediment budget by producing on average ten times more sediment than non-El Niño years. Sediment load estimates provided further evidence that antecedent hydrologic conditions appear to have caused much of the temporal dependence of suspended sediment behavior.

Rivers display temporal dependence in suspended sediment-water discharge relationships. Although most work has focused on multi-decadal trends, river sediment behavior often displays sub-decadal scale fluctuations that have received little attention. The objectives of this study were to identify inter-annual to decadal scale fluctuations in the suspended sediment-discharge relationship of a dry-summer subtropical river, infer the mechanisms behind these fluctuations, and examine the role of El Niño Southern Oscillation climate cycles. The Salinas River (California) is a moderate sized (11 000km2), coastal dry-summer subtropical catchment with a mean discharge (Qmean) of 11.6m3s-1. This watershed is located at the northern most extent of the Pacific coastal North America region that experiences increased storm frequency during El Niño years. Event to inter-annual scale suspended sediment behavior in this system was known to be influenced by antecedent hydrologic conditions, whereby previous hydrologic activity regulates the suspended sediment concentration-water discharge relationship. Fine and sand suspended sediment in the lower Salinas River exhibited persistent, decadal scale periods of positive and negative discharge corrected concentrations. The decadal scale variability in suspended sediment behavior was influenced by inter-annual to decadal scale fluctuations in hydrologic characteristics, including: elapsed time since small (~0.1×Qmean), and moderate (~10×Qmean) threshold discharge values, the number of preceding days that low/no flow occurred, and annual water yield. El Niño climatic activity was found to have little effect on decadal-scale fluctuations in the fine suspended sediment-discharge relationship due to low or no effect on the frequency of moderate to low discharge magnitudes, annual precipitation, and water yield. However, sand concentrations generally increased in El Niño years due to the increased frequency of moderate to high magnitude discharge events, which generally increase sand supply.