Flow depletion in irrigated western river basins
Over 80% of water in the western U.S. is used for agriculture, and thousands of stream miles are now chronically depleted over the summer low flow season to supply irrigation. Depleted streams are more susceptible to heating, and excessive temperatures are often cited as a population bottleneck for aquatic species. At the same time, in depleted streams, even small amounts of colder water such as lateral return flows from irrigation seepage can moderate temperatures and provide critical thermal refugia. To improve understanding of the role of irrigation diversions and lateral return flows on summer stream temperature patterns, we are monitoring flow and temperature along diversion-depleted stream reaches in northern Utah. Findings are being used to help natural resource managers identify priority locations for additional monitoring and in-stream water transfers.
The flow regime acts as a major control on geomorphic processes and river ecosystems. We have distinguished 9 natural stream classes and 11 alteration classes that are being used to organize regional environmental water management efforts for California.
Download the KMZ of the hydrologic classification:
Subreach-scale (<100 m) variations in channel width and depth have rarely been included in channel classifications. However, variability in topographic features of rivers, in conjunction with sediment supply and discharge, produces a mosaic of channel forms that provides unique habitats for sensitive aquatic species. We are investigating the utility of topographic variability attributes (TVAs) in distinguishing channel types and dominant geomorphic processes. Initial results indicate that incorporating TVAs in channel classification provides a quantitative basis for interpreting nonuniform as well as uniform geomorphic processes, which can improve our ability to distinguish linked channel forms and processes of geomorphic and ecological significance. These efforts are now being applied throughout the State of California through partnerships with several academic entities.
The significance of channel topographic variability
Regional Ecohydraulic Analysis
The extent and timing of many river ecosystem functions is controlled by the interplay of streamflow dynamics (flow) with the river corridor shape and structure (form). However, most regional river management studies evaluate these components independently. We are developing a flexible, integrated modeling approach to assess the distributed ecological effects (function) resulting from different flow and form configurations.
2D hydrodynamic modeling is used in combination with geomorphic classification and synthetic river corridors to evaluate alternative flow-form-function scenarios at management relevant scales. The use of synthetic, archetypal river models based on mathematical descriptions of geomorphic attributes derived from geomorphic classification in lieu of high-resolution topographic data (i) reduces resource requirements, (ii) overcomes site-specific topographic features, and (iii) allows for evaluation of any morphological structure of interest. Ecosystem performance is measured across a suite of functions related to aquatic species, riparian species, and hydrogeomorphic processes.
Initial results from this framework highlight critical performance tradeoffs and emphasize the significance of spatiotemporal diversity of flow and form at multiple scales for maintaining river ecosystem integrity. The approach is broadly applicable and extensible to other systems and ecosystem functions. We are currently using this framework to inform river management and design testing in western states.