Interactions Between Physical and Biological Processes in Riverine Landscapes V: Ecohydraulics A
Presiding: J M Buffington, USDA Forest Service; C V Baxter, Colorado State University; A E Rosenberger, University of Idaho and USDA Forest Service
B44B-01 15:30h
Bed mobility: A Key Linkage Between Channel Condition and Lotic Ecosystems
Bed mobility is a key linkage between the physical dynamics of stream channels and lotic and riparian ecosystems over a range of scale. The depth, extent, and frequency of mobilization of bed material affect the variation of conditions providing the requirements of life stages of organisms living in and along the stream channel. Bed mobility thereby helps to define habitat patches supporting communities of organisms. At a larger scale, the rate of channel evolution and creation of new channel and riparian surfaces are governed by the ability of the channel to deform during sediment transport. Bed mobility is partly regulated by the tendency of coarse particles to interact to form structures (e.g., steps, clusters, cells) and sort themselves into coarse surface layers that resist transport. The capacity of the arrangement of bed particles to influence bed mobility generally declines with channels that have gentler slopes and better sorted (finer) bed material. But within channels having a common range of slope and bed material size, bed mobility can adjust to temporary changes in the caliber and rate of sediment supply. Thus bed mobility can be used as an integrative measure of physical habitat condition. Classification of the mobility of the surface of a gravel-bed stream channel is offered as an organizing framework to examine the influence of sediment supply and flow on benthic habitats. Degrees of bed activity can be related to particular `mobility fields' that are quantified by Shields stress (ratio of impelling to resisting forces acting on bed particles). Mobility fields are classified on the basis of the fraction of surface particles moved and the depth of bed mobilization, which limit the scale and severity of bed disturbance. As an approximation, the surface remains stable at Shields<0.03, is partially mobile at 0.03<Shields<0.06, fully mobile at 0.06<Shields<0.12, and unarmored at Shields>0.12. Only fully mobile beds are capable of significant evolution, and greater mobility is associated with channel instability. The proportion of patches having different degrees of mobility governs the dynamics of habitats at the reach scale. Bed mobility could be useful in evaluating channel condition and predicting channel stability following restoration.
B44B-02 15:45h
The Influence of Relative Sediment Supply on Riverine Habitat Heterogeneity
The diversity of aquatic habitats in a stream reach and associated biological response are linked to physical processes that act at various spatial and time scales within a watershed. A fundamental driver of stream geomorphology integrating multi-scale processes is the relationship between sediment supply and transport capacity. This study explored the interactions between riverine habitat heterogeneity and the geomorphic processes governing channel conditions by testing the hypothesis that maximum habitat heterogeneity occurs in stream reaches with a moderate relative local sediment supply, as measured by the supply-capacity ratio. Habitat heterogeneity was quantified with an ecologically meaningful spatial heterogeneity index from the field of landscape ecology, Shannon's Diversity Index (SHDI), and relative sediment supply was quantified using a dimensionless bedload transport rate, q*. Data from previously published studies and a new field study on tributaries to the South Yuba River in Nevada County, California were evaluated to test the hypothesis. Results showed that in alluvial reaches where flow and sediment interacted freely without obstruction, moderate q* values correlated with high SHDI values; however, in reaches where less mobile structural elements, such as large woody debris and boulders, were present, SHDI increased as the percentage of structural elements increased. The results indicate two potential mechanisms for how relative sediment supply may drive habitat diversity at the reach scale. When structural elements are not a large proportion of the reach landscape, the supply-capacity ratio dictates the range of sediment textures and geomorphic features observed such that channels with a moderate relative sediment supply exhibit high habitat heterogeneity supporting the study hypothesis. In contrast, when structural elements are relatively abundant, increased local scour and deposition creates a greater variety of geomorphic features and sorted sediment textures, thereby increasing the habitat heterogeneity observed. For both mechanisms, it is the combination of variable sediment flux and variable flow magnitude and frequency that creates the variations in sediment mobility required to maximize geomorphic diversity.
B44B-03 16:00h
Interactions between fine sediment infiltration, interstitial flow and salmonid embryo survival in contrasting gravel-bed rivers
Fine sediment infiltration into salmon spawning gravels is widely recognised as a factor limiting recuitments and productivity. Considerable effort has been focussed on developing empircal and theoretical links between fine sediment and the survival of salmonid embryos. This paper presents the results of a field research programme undertaken in four contrasting salmonid spawning streams. At each site measurement of fine sediment accumulation within artifical redds was made simultaneously with dissolved oxygen, intragravel flow rate and water temperature. At the end of the study egg survival was determined from within each redd. The results demonstrate a complex relationship between egg survival and common metrics of fine sediment accumulation. A conceptual model is developed that demonstrates the composite of factors determining egg survival at a given site. It is suggested that at least within the study streams, use of simple sediment metrics is of limited use in determining spawning habitat quality.
http://www.geog.soton.ac.uk/research/maff/DEFRA_Final.pdf
B44B-04 16:15h
Interactions of geomorphic process and form associated with Chinook salmon spawning habitat on the Yuba River, northern California.
The study identifies the links between channel form, physical process and habitat utilisation at a site on the Yuba River that annually experiences high levels of spawning activity by chinook salmon (Oncorhynchus tshawytscha). Such relationships control the provision of suitable habitat conditions and are responsible for the longer-term maintenance and evolution of the geomorphic features that support spawning. Data of this nature are essential to provide the basis for sustainable rehabilitation designs that are sympathetic to natural geomorphic and ecological processes. An initial assessment linked a 2-D hydrodynamic, sediment entrainment, and physical habitat model of the site resolved at the micro-habitat scale (0.1-1.0 m, the scale at which fish actually experience a river) with over 400 redd positions. At the micro-habitat scale, the model described the hydraulic (i.e. depth, velocity, Froude number, shear stress) environment associated with spawning site utilisation. Habitat suitability indices (HSIs) for spawning Chinook salmon were applied to model output to predict habitat availability that was then compared to actual redd distributions at the site. Micro-habitat scale information was then nested within a 10-100 m scale geomorphic context to identify discrete hydraulic-morphological habitat types, assess cross-section geomorphic conditions, and predict sediment transport rates. Using the 2D model and tracer experiments, information of sediment mobility over a range of discharges provided an indication of the processes that control the sedimentology of the site and, ultimately, the distribution of habitat.
B44B-05 16:30h
Landscape Pattern, Network Structure, and the Distribution of Coastal Cutthroat Trout in Headwater Streams
Headwater streams are dynamic environments in which landscape characteristics exert strong influences on the distribution of stream fishes. Although geology, topographic factors, and land use have been shown to affect trout population density at a site-specific level, few studies have investigated landscape features associated with the spatial extent and scale of variation of trout distribution in an entire watershed. To evaluate landscape influences on the distribution and abundance of coastal cutthroat trout (Oncorhynchus clarki clarki), we conducted spatially continuous surveys of stream habitat and trout abundance in forty randomly selected watersheds (500-1000 ha) in the Cascades, Coast Range, and Klamath Mountains ecoregions of western Oregon. Our investigation of coastal cutthroat trout populations across a broad range of headwater environments revealed that landscape features, including topography, geology, network structure, annual precipitation, and stand replacement disturbance, were associated with the spatial extent and scale of variation of trout distribution within watersheds. Understanding effects of basin-scale factors on trout distribution and abundance is critical in forested regions such as the Pacific Northwest where resource managers must consider potential impacts of logging on aquatic ecosystems.
http://oregonstate.edu/~torgersc/
B44B-06 16:45h
Spatial Hydraulic Metrics: Linking Channel Morphology, Flow Complexity and Stream Habitat
Channel morphology is driven by a complex interaction between a watershed's hydrologic regime, geographic features, and sediment transport processes. Moreover, the geomorphic characteristics of streams may vary within different watersheds as well as along the stream itself. Individual morphologic features such as channel meanders, bars, pool-riffle sequences, as well as exposed rocks/outcrops in turn create numerous and varied types of flow complexity within channels. Evidence suggests that flow complexity is an important component of habitat for various aquatic organisms. Existing methods of quantifying hydraulic habitat typically rely upon one-dimensional hydraulic models to quantify the depths and velocities throughout a channel reach. Whereupon, point metrics (e.g. depth and velocity) are then used to determine the locations and amount of suitable hydraulic habitat within the stream reach. Such an approach is not capable of accurately reproducing flow complexity and assumes that two locations having similar depth and velocity values are equally suitable/unsuitable habitat, regardless of the flow conditions surrounding those two points. Methods for quantifying geomorphic features, the flow features they create, and the aquatic organisms using/preferring these habitats are needed to guide watershed/stream restoration projects. A conceptual model for quantifying how the flow features created by geomorphic features influence the amount and quality of habitat within a stream is presented. Specifically, it is proposed that after conducting a detailed survey of a channel reach, a multi-dimensional (2- or 3-dimensional) hydraulic model can be employed to model the hydraulic habitat (including flow complexities of biological importance). Spatial hydraulic metrics are then utilized to quantify flow complexity within the stream. Examples of implementing two-dimensional hydraulic models and spatial hydraulic metrics to model flow complexity within two-stream reaches are provided.