H34A-01 INVITED
Conceptualizing and Communicating River Restoration
River restoration increasingly involves collaboration with stakeholders having diverse values and varying technical understanding. In cases where river restoration proceeds through collaborative processes, scientists are required to communicate complex understanding about riverine ecosystem processes to broad audiences. Of particular importance is communication of uncertainties in predictions of ecosystem responses to restoration actions, and how those uncertainties affect monitoring and evaluation strategies. I present a relatively simple conceptual model of how riverine ecosystems operate. The model, which has been used to conceptualize and communicate various river-restoration and management processes in the Lower Missouri River, emphasizes a) the interdependencies of driving regimes (for example, flow, sediment, and water quality), b) the filtering effect of management history, c) the typical hierarchical nature of information about how ecosystems operate, and d) how scientific understanding interacts with decision making. I provide an example of how the conceptual model has been used to illustrate the effects of extensive channel re-engineering of the Lower Missouri River which is intended to mitigate the effects of channelization and flow regulation on aquatic and flood-plain ecosystems. The conceptual model illustrates the logic for prioritizing investments in monitoring and evaluation, interactions among ecosystem components, tradeoffs between ecological and social-commercial benefits, and the feedback loop necessary for successful adaptive management.
H34A-02
An Assessment of Pulsed Flows on Foothill Yellow-legged Frog Habitat Hydraulics in a Regulated River using Two-Dimensional Hydrodynamic Modeling
We used a freely available two-dimensional model, River2D, to evaluate changes in habitat suitability and availability for Foothill yellow-legged frog egg masses and tadpoles during pulsed flow events. Two study sites in Northern California, one on the unregulated South Fork Eel River and the other on the regulated North Fork Feather River, were selected for modeling. Simulated depths and velocities agreed well with measured field values. When coupled with a definition of breeding habitat suitability that encompassed the variability of field- measured values and the range of error within the model output, the model accurately predicted suitable breeding locations throughout the survey reach. Using data on percentages of egg mass and tadpole loss associated with increased velocities, we assessed several scenarios of how pulsed flows affected habitat availability and suitability. In a seasonal (spring) pulse scenario, lower discharges provided the greatest weighted usable area for breeding, but higher initial discharges provided the greatest buffering capacity against lethal increases in velocity. In an aseasonal (summer) pulse scenario, only 20-30% of the suitable tadpole habitat in the unregulated site and <5% of the suitable habitat in the regulated site remained suitable during the pulse regardless of initial flow level. In both scenarios, the unregulated study site provided 2-3 times the buffering capacity of the regulated site. This was likely due to differences in channel morphology; the regulated site had an entrenched channel with steep banks, while the unregulated site had an asymmetric cross-sectional shape where shallow overbank areas provided refuge from high velocities as flows fluctuated. This type of model-based methodology that can evaluate effects from flow fluctuation on individuals and local habitat conditions for multiple life stages would be useful for managing Foothill yellow-legged frog or similar aquatic species in regulated river systems.
H34A-03
Conditions Necessary to Create and Maintain Meandering Channels: Inferences From Flume Experiments
Stream restoration projects often involve constructing single-thread meandering channels, and their success requires understanding the conditions necessary to develop and maintain a meandering pattern. Empirical studies indicate that meandering channels occur under a specific range of width-depth ratio, slope and Froude number. We hypothesize, however, that meandering rivers also require: 1) bank strength from either cohesive material or vegetation, 2) overbank flows to attach bars to their floodplains, and 3) fine sediment to fill the downstream end of bars and chutes. These latter conditions place significant additional constraints in gravel bedded channels, especially ones designed to be self-maintaining while laterally shifting. We tested whether these conditions were sufficient in a sand-bedded laboratory flume using alfalfa as model vegetation and a non- cohesive lightweight plastic as fine sediment. We conducted two experiments in a 6.1 m wide and 17 m long basin with a valley slope of 0.0046. The first experiment was conducted with a two-stage hydrograph with a bankfull flow of 1.8 l/s and an overbank flow of 2.7 l/s. Prior to the first experiment, we carved a channel with an initial geometry of 1.9 cm deep and of 40 cm wide. The second experiment began with the final morphology from the first experiment, and had a steady 1.8 l/s flow. We were able to create and maintain a meandering channel during both experiments. The channel maintained a meandering morphology for over 71 hours during the first experiment and an additional 35 hours to date during the steady experiment without the channel braiding. The alfalfa sprouts slowed down bank erosion enough to allow bars to grow to the elevation of the floodplain, and the fine sediment plugged chutes at the upstream end of bars. During the first experiment, the width initially increased, but then stabilized as the bars grew vertically. The width depth ratio had returned to its original value (21) by 51 hours, just prior to a cutoff that forced more water overbank and increased the width-depth ratio to 25. During the steady flow experiment, the channel reduced its flow capacity so that the discharge resulted in a steady overbank flow, and the sinuosity increased relative to the first experiment. This prevented us from testing whether overbank flows were necessary, but did show that a meandering morphology can be maintained with a steady flow. These experiments indicate that bank strength (here derived from vegetation) and fine sediment are essential components of meandering channels in the laboratory and should likely be incorporated into channel design.
H34A-04
Using Mechanistic Studies to Model Riparian Tree Establishment Under Environmental Flow Scenarios on Regulated Rivers
In the Central Valley of California, pioneer cottonwood and willow species dominate the near-river forests. Historically, seedling recruitment for these disturbance-adapted species coincided with spring floods. Changes in flow timing and magnitude due to river regulation have decreased the success of seedling cohorts and contributed to the decline of these riparian tree populations. In order to address gaps in our understanding of these species and potential restoration strategies, we field-calibrated a conceptual model of seedling recruitment for the dominant pioneer woody species, Populus fremontii, Salix gooddingii, and S. exigua. We conducted experiments to identify seedling desiccation thresholds and seed longevity, used field studies to measure seedling competition and seasonal seed release patterns, and modeled interannual differences in dispersal timing using a degree-day model. These studies were integrated into a recruitment model that generates annual estimates of seedling density and bank elevation based on inputs of seasonal river discharge, seed dispersal timing, and seedling mortality from desiccation. The model predictions successfully captured interannual and species-level patterns in recruitment observed independently throughout a 20-km reach of the lower Tuolumne River from 2002-04. The model correctly predicted that seedling densities were highest in 2004 and lowest in 2003, and that S. exigua recruitment would be less extensive than for the two tree species. This work shows promise as both a quantitative approach linking hydrology, climate and plant community dynamics, and as a process-based framework for guiding flow releases and other management actions to restore riparian tree population along Central Valley rivers.
H34A-05
Controlled Floods, Geomorphic Processes, and Riparian Vegetation Mortality in a Sand-Bed Desert River
We investigate interactions among flow, geomorphic processes, and riparian vegetation using field studies and computational modeling in an effort to improve both process understanding and the quantitative basis of ecosystem flow prescriptions. We examined the response of native and non-native riparian seedlings and channel morphology to prescribed dam-released floods to maintain a Populus-Salix riparian woodland system on the Bill Williams River, Arizona, USA. Floods in consecutive years produced substantial density reductions among cohorts of one-year old seedlings in two study reaches, with greater reductions among non- native Tamarix seedlings than native Salix seedlings. Little or no mortality occurred among more mature vegetation, however, in part because of the greater drag produced by larger plants. Mechanisms of seedling mortality differed as a function of reach-scale channel morphology and potentially basin-scale sediment supply, with scour-related mortality occurring in a sediment-starved reach and aggradation and burial-induced mortality occurring in a downstream reach with greater sediment supply. We also assessed spatially distributed velocity, shear stress, and sediment mobility associated with floods on the Bill Williams River using the US Geological Survey Multidimensional Surface Water Modeling System and compared the modeled hydraulics to observed seedling responses. Our results illustrate that in sand-bed rivers, even relatively small floods generate sufficient hydraulic forces and geomorphic changes to substantially influence seedling establishment and mortality. Further, controlled flood releases to reduce Tamarix density before seedlings become established (i.e., in their 1st year) may be a method of managing streamflow to provide natives with a competitive advantage over Tamarix. Combining field studies with multi-dimensional hydraulic modeling provides a means of quantifying the potential geomorphic and ecological effects of high flows and therefore can assist development of flow prescriptions to achieve restoration goals.
H34A-06
A tale of two rivers: channel adjustments to restorative floods in the Green River in Dinosaur N.M. as compared to those in the Colorado River in Grand Canyon N.P.
Sediment mass balance is a critical system attribute in assessing the potential for restoration of dam-impacted rivers. We compared channel response to large floods on the Green River in Lodore Canyon to similar changes measured along the Colorado River in part of Grand Canyon National Park, a reach with similar geomorphic organization, regulatory constraints, and habitat management goals. The post-dam sediment mass balance of the Green River is indeterminate or in surplus, but the mass balance of the Colorado River is in deficit. Analysis of repeat measurements at 36 cross sections along a 20 km reach of Lodore Canyon show that the sand storage condition in 2006 was no different than the condition observed in 1994, despite an increased frequency of high magnitude floods. Four high magnitude floods occurred in 1997, 1999, 2005, and 2006, but only one, the 1999 flow, triggered channel adjustments to the bed and banks that were significantly different than those of the post- dam 2-year return flood. This condition of relative equilibrium contrasts the sand storage condition of the Colorado River in Grand Canyon, where sand bar area and volume have declined despite specific dam releases intended to rebuild sand bars. The contrasting patterns of channel adjustment in these rivers indicate that the opportunities and cost of restoration are likely to differ in relation to the sediment supply available for channel restoration.
H34A-07
Limits on characteristics of invertebrate assemblages associated with streamflow patterns in the western United States
River restoration depends on re-establishment of the range of physical and biological processes that comprise the river ecosystem. Streamflow is the definitive physical processes for river ecosystems, so hydrologic alteration represents a potentially significant issue to be addressed by restoration efforts. Given adaptation of lotic species to naturally variable streamflow patterns over evolutionary time scales, however, lotic communities are resilient to at least some forms of hydrologic variability. As a result, river restoration may be successful despite limited but biologically insignificant hydrologic alteration. The responses of benthic invertebrate assemblages to variation in streamflow patterns across the western United States were investigated to identify biologically important forms and magnitudes of hydrologic variability. Biological responses to streamflow patterns were analyzed in terms of ceilings and floors on invertebrate assemblage diversity and structure using a non-parametric screening procedure and quantile regression. Variability at daily and monthly time scales was the most common streamflow pattern associated with broad metrics of invertebrate assemblages including abundance; richness and relative abundance of Ephemeroptera, Plecoptera, Trichoptera and non-insects; dominance; and diversity. Low flow magnitude and annual variability were associated with richness and trophic structure. The frequency, magnitude, and duration of high flows were associated with abundance and richness. Longer term streamflow metrics (calculated over at least 5 years) were more important than recent flows (30 and 100 days prior to invertebrate sampling). The results can be used as general guidance about when hydrologic alteration is likely to be an important factor and what streamflow patterns may need to be re-established for successful river restoration.