Impact of flow regulation on stream morphology and habitat quality distribution.

The importance of interactions among stream hydrology, morphology, and biology is well recognized in studies of stream ecosystems. However, when quantifying the impacts of altered flow on aquatic habitat, results are often based either on combined changes in topography and flow, or with altered flow over static topography. Here, we study the potential beneficial effects of restoring unregulated flows on salmonid habitat and separate the relative influences of changes in flow vs. topography. We hypothesize that flow restoration will increase topographic complexity and that the coevolution of topography with altered streamflow will produce stronger changes in habitat than predicted for static topography. We address this hypothesis by quantifying spawning and juvenile rearing habitat distributions for Chinook salmon (Oncorhynchus tshawytscha) from a set of quasi-three-dimensional hydromorphodynamic models for two morphologically distinct reaches along the Lemhi River, Idaho (USA): an engineered, straightened, plane-bed reach, and a less-altered, meandering, pool-riffle reach. Sediment transport was modeled with hydrographs predicted for actual interannual variability of flow and for a synthetic annual flow representing the ensemble actual hydrographs for 60 years of regulated and unregulated flows. The actual and synthetic hydrographs predicted from the model produced similar morphologic results, which implies that interannual flow variation and hydrograph order did not have a strong effect on the modeled topography. Unregulated hydrographs enhanced the geometry and frequency of pools in the meandering reach compared to regulated flows. These morphological changes did not increase habitat quality predicted from suitability indices, but the large growth of pools likely improved winter refugia for juvenile salmon. In the straight reach, both regulated and unregulated scenarios resulted in a plane-bed morphology, suggesting that flow restoration in highly altered reaches is not sufficient to improve ecological function.

Development of a spatially-distributed hydroecological model to simulate cottonwood seedling recruitment along rivers.

Dam operations have altered flood and flow patterns and prevented successful cottonwood seedling recruitment along many rivers. To guide reservoir flow releases to meet cottonwood recruitment needs, we developed a spatially-distributed, GIS-based model that analyzes the hydrophysical requirements for cottonwood recruitment. These requirements are indicated by five physical parameters: (1) annual peak flow timing relative to the interval of seed dispersal, (2) shear stress, which characterizes disturbance, (3) local stage recession after seedling recruitment, (4) recruitment elevation above base flow stage, and (5) duration of winter flooding, which may contribute to seedling mortality. The model categorizes the potential for cottonwood recruitment in four classes and attributes a suitability value at each individual spatial location. The model accuracy was estimated with an error matrix analysis by comparing simulated and field-observed recruitment success. The overall accuracies of this Spatially-Distributed Cottonwood Recruitment model were 47% for a braided reach and 68% for a meander reach along the Kootenai River in Idaho, USA. Model accuracies increased to 64% and 72%, respectively, when fewer favorability classes were considered. The model predicted areas of similarly favorable recruitment potential for 1997 and 2006, two recent years with successful cottonwood recruitment. This model should provide a useful tool to quantify impacts of human activities and climatic variability on cottonwood recruitment, and to prescribe instream flow regimes for the conservation and restoration of riparian woodlands.