River Restoration From Boundary Layer to Watershed: Integrating Physical and Biological Science Over Space and Time IV
Presiding: P Wilcock, Applied Physics Laboratory, Johns Hopkins University; M Power, University of California, Berkeley
NB23F-01 INVITED 13:30h
Coupling Hydrodynamic Modeling and Empirical Measures of Bed Mobility: Implications for Restoring Spawning Gravel Quality on a Large Regulated River
Flow releases are increasingly being used as a tool to restore spawning gravel quality downstream of large dams. Often times, the primary goal of a peak flow release is to flush fine sediment from incubation habitat and restore active river processes; thus understanding the portion of the bed that is entrained and the flow required for full mobility is important. However, a critical knowledge gap for implementing these experimental floods is predicting the potential scouring of spawning redds in downstream reaches. To address these questions we need to understand the relationships between river discharge, bed mobility, and scour depths in areas heavily utilized by spawning salmon. Our approach couples numerical flow modeling and empirical data to quantify spatially explicit zones of differential bed mobility and identify specific areas where scour is deep enough to impact redd viability. Boundary shear stress values were predicted using the USGS's Multi-Dimensional Surface Water Modeling System for a segment of the Trinity River below Lewiston Dam. From model-generated shear stress and fine-scale mapping of local particle size distributions, Shields stress values were calculated to identify areas of differential bed mobility. Our data suggest that full mobility is limited to a central, yet discontinuous core along the thalweg, which expands with increasing flow strengths. Spatial analysis revealed that Chinook salmon tend not to spawn in areas that became fully mobile during bankfull flood events. Scour depths in areas preferentially used for spawning were less than in other portions of the bed and were not deep enough to impact incubating eggs or embryos. This site-selection preference allows fish to spawn in areas that are relatively safe from deep scour. However, the trade-off for spawning in stable areas is that flushing of fine sediment from the subsurface occurs very infrequently and gravel permeability is likely to become limiting for egg and embryo survival. Extremely large floods will be required to flush fine sediment stored in the subsurface from areas preferentially used for spawning. Preliminary evidence also suggests that the increase in gravel quality associated with the flushing of fines from the subsurface (by mobilization of the bed during high flow events or by fish during the excavation of redds) is short lived because of the large influx of fine bed material from tributaries. This study indicates that a combined strategy of reducing fine sediment inputs and providing peak flows large enough to mobilize substantial portions of the bed but small enough not to scour incubating embryos is needed to maintain favorable spawning and incubation habitat below reservoirs. The optimum range of peak flows can be designed with flow models that predict areas of mobility in relation to areas selectively used by spawning fish.
NB23F-02 13:45h
Contrasting Patterns of Juvenile Chinook Salmon (Oncorhynchus tshawytscha) Growth, Diet, and Prey Densities in Off-channel and Main Channel Habitats on the Sacramento River.
Few studies have quantified juvenile salmon growth in or between different habitats or evaluated the mechanisms by which salmon growth and survival might be enhanced. We used otolith microstructure to compare daily relative growth rates among main channel areas, off-channel ponds, and non-natal seasonal tributaries of the Sacramento River in 2001 and 2002. To examine possible mechanisms leading to growth differences, prey availability, prey preference, and stomach fullness were estimated at each site. Stable isotope ratios (Δ13C and Δ15N) in salmon tissue and their predominant prey were measured in 2002. We observed wider daily increment widths, higher prey densities, and warmer temperatures in off-channel ponds and non-natal seasonal tributaries in both 2001 and 2002. Off-channel pond salmon and chironomidae pupae had significantly different Δ13C and Δ15N than those captured in the main channel and non-natal seasonal tributaries. In 2001, all habitats had higher temperatures, wider daily increment widths, higher prey densities, and higher stomach fullness than in 2002. Our findings suggest warmer temperatures and abundant prey in off-channel habitats lead to higher growth rates. Increased access to off-channel habitats during wetter years may account for the stronger year classes and higher survival rates reported in other studies.
NB23F-03 14:00h
Integrating Ecology and Geomorphology in Etowah River Shoal Restoration
Designs of stream restoration projects are typically based primarily on geomorphic data. However, these data may not be comprehensive enough to design ecologically successful restoration projects. In summer 2004 Georgia Ecological Services (U.S. Fish and Wildlife Service) surveyed nearly 80 km of the Etowah River, GA. The goal of this survey was to identify sediment sources and potential restoration techniques that would benefit imperiled, shoal-inhabiting fishes in the Etowah River. An essential and unique part of this survey was to collect both ecological and geomorphic data to build a more holistic understanding of how and why shoal habitats vary. One ecological measure included was the density and length of river weed Podostemum ceratophyllum, a submerged aquatic macrophyte. Other studies have shown that the imperiled "Coosa" madtom (Noturus sp. cf. N. munitus) and freckled darter (Percina lenticula) occur more frequently in the presence of Podostemum. Results indicate that the density and length of Podostemum increases with shoal width and particle size, highlighting the importance of geomorphology in its growth. Restoration activities that focus on these habitat characteristics will facilitate Podostemum growth and will probably aid in the recovery of the "Coosa" madtom and freckled darter in the Etowah River.
NB23F-04 14:15h
Effects of In-stream Restorations on Stream Hydrodynamics, Nutrient Uptake, and Ecosystem Metabolism at Fort Benning, GA
Spatial variability in military training intensity results in a wide range of upland disturbance intensity at the Fort Benning Military Reservation (near Columbus, GA). We selected stream reaches within 8 catchments with contrasting levels of upland denudation and stream ecosystem disturbance. In October 2003, 4 of these streams (spanning the disturbance gradient) received in-stream restorations in the form of coarse woody debris dams every 10 m for the 100 m study reaches. Stream hydrodynamic properties, NH4+ uptake, and whole-stream metabolism were examined both prior to and after restoration for all 8 streams. In-stream restorations resulted in increases in the relative size of transient storage zones (important for biological processes) and spatial variation in water velocity (enhances habitat variability). These hydrodynamic changes corresponded to increases in both NH4+ uptake rate and velocity as the ability of stream biota to control stream NH4+ concentration increased. By monitoring stream metabolism rates for two years prior to restoration we are able to assess the impact of the restorations on these important integrative processes using a BACI-type analysis (before-after control-treatment analysis) for the first year of post-restoration.
NB23F-05 INVITED 14:30h
How Riparian Vegetation Influences Stream Morphology and Process: Implications for Restoration
Vegetation is often used to enhance bank stabilization efforts and as part of more generalized restoration designs, but the influence of vegetation on geomorphic processes is poorly understood. Recent research suggests that changes in riparian vegetation can influence channel morphology, rates of erosion and deposition, and by extension, the entire sediment budget of a reach. In recent studies in the Mid-Atlantic region, reaches with forested riparian vegetation were compared to immediately adjacent non-forested (grassy) riparian vegetation to explain why channels with forested vegetation are wider than their non-forested counterparts. At study sites, erosion occurs at cutbanks in curving reaches, while deposition is localized on active floodplains at the insides of bends. In narrow, non-forested reaches, rates of bank erosion and channel migration are high, while in wide, forested reaches, rates of bank erosion and channel migration are low. These results may be explained using the ratio of two dimensionless parameters, alpha and E. Alpha represents the influence of vegetation on rates of active floodplain deposition (it is very high in non-forested reaches, and very low in forested reaches), while E is proportional to rates of cutbank migration (it is high in non-forested areas and low in forested areas). The width is proportional to E/alpha. These results could lead to improved assessment of the geomorphic effects of restoration designs that involve changes in riparian vegetation.