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Geomorphic effectiveness of floods varied temporally as plants grew largerFloods killed small woody seedlings as a result of both bed scour and burialFloods produced greater seedling mortality among tamarisk than willow Interactions among flow, geomorphic processes, and riparian vegetation can strongly influence both channel form and vegetation communities. To investigate such interactions, we took advantage of a series of dam‐managed flood releases that were designed in part to maintain a native riparian woodland system on a sand‐bed, dryland river, the Bill Williams River, Arizona, USA. Our resulting multiyear flow experiment examined differential mortality among native and nonnative riparian seedlings, associated flood hydraulics and geomorphic changes, and the temporal evolution of feedbacks among vegetation, channel form, and hydraulics. We found that floods produced geomorphic and vegetation responses that varied with distance downstream of a dam, with scour and associated seedling mortality closer to the dam and aggradation and burial‐induced mortality in a downstream reach. We also observed significantly greater mortality among nonnative tamarisk (Tamarix) seedlings than among native willow (Salix gooddingii)seedlings, reflecting the greater first‐year growth of willow relative to tamarisk. When vegetation was small early in our study period, the effects of vegetation on flood hydraulics and on mediating flood‐induced channel change were minimal. Vegetation growth in subsequent years resulted in stronger feedbacks, such that vegetation's stabilizing effect on bars and its drag effect on flow progressively increased, muting the geomorphic effects of a larger flood release. These observations suggest that the effectiveness of floods in producing geomorphic and ecological changes varies not only as a function of flood magnitude and duration, but also of antecedent vegetation density and size.

Beaver convert lotic stream habitat to lentic through dam construction, and the process is reversed when a flood or other event causes dam failure. We investigated both processes on a regulated Sonoran Desert stream, using the criterion that average current velocity is < 0·2 m s−1in a lentic reach. We estimated temporal change in the lotic:lentic stream length ratio by relating beaver pond length (determined by the upstream lentic–lotic boundary position) to dam size, and coupling that to the dam‐size frequency distribution and repeated censuses of dams along the 58‐km river. The ratio fell from 19:1 when no beaver dams were present to < 3:1 after 7 years of flows favourable for beaver. We investigated the dam failure–flood intensity relationship in three independent trials (experimental floods) featuring peak discharge ranging from 37 to 65 m3s−1. Major damage (breach ≥ 3‐m wide) occurred at ≥ 20% of monitored dams (n= 7–86) and a similar or higher proportion was moderately damaged. We detected neither a relationship between dam size and damage level nor a flood discharge threshold for initiating major damage. Dam constituent materials appeared to control the probability of major damage at low (attenuated) flood magnitude. We conclude that environmental flows prescribed to sustain desert riparian forest will also reduce beaver‐created lentic habitat in a non‐linear manner determined by both beaver dam and flood attributes. Consideration of both desirable and undesirable consequences of ecological engineering by beaver is important when optimizing environmental flows to meet ecological and socioeconomic goals. Copyright © 2010 John Wiley & Sons, Ltd.

Concern about spread of non-native riparian trees in the western USA has led to Congressional proposals to accelerate control efforts. Debate over these proposals is frustrated by limited knowledge of non-native species distribution and abundance. We measured abundance of 44 riparian woody plants at 475 randomly selected stream gaging stations in 17 western states. Our sample indicates that Tamarix ramosissima and Elaeagnus angustifolia are already the third and fourth most frequently occurring woody riparian plants in the region. Although many species of Tamarix have been reported in the region, T. ramosissima (here including T. chinensis and hybrids) is by far the most abundant. The frequency of occurrence of T. ramosissima has a strong positive relation with the mean annual minimum temperature, which is consistent with hypothesized frost sensitivity. In contrast the frequency of occurrence of E. angustifolia decreases with increasing minimum temperatures. Based on mean normalized cover, T. ramosissima and E. angustifolia are the second and fifth most dominant woody riparian species in the western USA. The dominance of T. ramosissima has been suspected for decades; the regional ascendance of E. angustifolia, however, has not previously been reported.

Human demands for surface and shallow alluvial groundwater have contributed to the loss, fragmentation, and simplification of riparian ecosystems. Populus species typically dominate riparian ecosystems throughout arid and semiarid regions of North American and efforts to minimize loss of riparian Populusrequires an integrated understanding of the role of surface and groundwater dynamics in the establishment of new, and maintenance of existing, stands. In a controlled, whole-stand field experiment, we quantified responses of Populusmorphology, growth, and mortality to water stress resulting from sustained water table decline following in-channel sand mining along an ephemeral sandbed stream in eastern Colorado, USA. We measured live crown volume, radial stem growth, annual branch increment, and mortality of 689 live Populus deltoidessubsp. moniliferastems over four years in conjunction with localized water table declines. Measurements began one year prior to mining and included trees in both affected and unaffected areas. Populusdemonstrated a threshold response to water table declines in medium alluvial sands; sustained declines ≥1 m produced leaf desiccation and branch dieback within three weeks and significant declines in live crown volume, stem growth, and 88% mortality over a three-year period. Declines in live crown volume proved to be a significant leading indicator of mortality in the following year. A logistic regression of tree survival probability against the prior year's live crown volume was significant (−2 log likelihood = 270, χ2with 1 df= 232, P< 0.0001) and trees with absolute declines in live crown volume of ≥30 during one year had survival probabilities <0.5 in the following year. In contrast, more gradual water table declines of ~0.5 m had no measurable effect on mortality, stem growth, or live crown volume and produced significant declines only in annual branch growth increments. Developing quantitative information on the timing and extent of morphological responses and mortality of Populusto the rate, depth, and duration of water table declines can assist in the design of management prescriptions to minimize impacts of alluvial groundwater depletion on existing riparian Populusforests.Human demands for surface and shallow alluvial groundwater have contributed to the loss, fragmentation, and simplification of riparian ecosystems. Populus species typically dominate riparian ecosystems throughout arid and semiarid regions of North American and efforts to minimize loss of riparian Populusrequires an integrated understanding of the role of surface and groundwater dynamics in the establishment of new, and maintenance of existing, stands. In a controlled, whole-stand field experiment, we quantified responses of Populusmorphology, growth, and mortality to water stress resulting from sustained water table decline following in-channel sand mining along an ephemeral sandbed stream in eastern Colorado, USA. We measured live crown volume, radial stem growth, annual branch increment, and mortality of 689 live Populus deltoidessubsp. moniliferastems over four years in conjunction with localized water table declines. Measurements began one year prior to mining and included trees in both affected and unaffected areas. Populusdemonstrated a threshold response to water table declines in medium alluvial sands; sustained declines ≥1 m produced leaf desiccation and branch dieback within three weeks and significant declines in live crown volume, stem growth, and 88% mortality over a three-year period. Declines in live crown volume proved to be a significant leading indicator of mortality in the following year. A logistic regression of tree survival probability against the prior year's live crown volume was significant (−2 log likelihood = 270, χ2with 1 df= 232, P< 0.0001) and trees with absolute declines in live crown volume of ≥30 during one year had survival probabilities <0.5 in the following year. In contrast, more gradual water table declines of ~0.5 m had no measurable effect on mortality, stem growth, or live crown volume and produced significant declines only in annual branch growth increments. Developing quantitative information on the timing and extent of morphological responses and mortality of Populusto the rate, depth, and duration of water table declines can assist in the design of management prescriptions to minimize impacts of alluvial groundwater depletion on existing riparian Populusforests.