Eric W. Larsen

Measuring and Modeling the Hydraulic Environment for Assessing Instream Flows

Abstract Detailed measurements of water depth and velocity in natural channels, though rare, show that the velocity fields are complex and irregular even in streams with moderate gradients and gravel substrates. This complexity poses a challenge for instream flow studies, most of which use the physical habitat simulation (PHABSIM) model, a set of computer models that combine the results of hydraulic modeling with estimates of channel substrate or cover and habitat suitability criteria to compute weighted usable area (WUA), an index of habitat. Some recent studies have replaced the transect-based one-dimensional hydraulic modeling in PHABSIM with two-dimensional models that allow better definition of the depth and velocity fields in the modeled stream reach. The accuracy of the estimates as a function of channel geometry and data collection effort remains unclear, however, as does the utility of the estimates for evaluating instream flow needs. Here we review the assumptions, accuracy, and precision of hyd...

SIMULATED CHANGES IN SHALLOW GROUNDWATER AND VEGETATION DISTRIBUTIONS UNDER DIFFERENT RESERVOIR OPERATIONS SCENARIOS

The objectives of this study were to develop and use a linked groundwater and vegetation model to simulate groundwater and vegetation distributions in a riverine and reservoir-fringe system under different reservoir operations scenarios. This study was conducted where Little Stony Creek flows into East Park Reservoir on the east front of the Coast Range, northern California. A numerical groundwater model was used to model mean depth to groundwater during the growing season for water years 1980-1999 for each of five community types identified on the study site. Multiple vegetation models were devel- oped, each of which described the probability that a given community type would occur primarily as a function of modeled mean depth to groundwater during the growing season and secondarily as a function of flooding. Four scenarios representing four different res- ervoir operations were simulated: existing condition, existing condition with late drawdown, full drawdown, and full pool. A groundwater backwater effect caused by the imposed reservoir stage extends to portions of the terrace, but the most pronounced effects occur on the delta. Consequently, the most pronounced changes in vegetation distributions also occur on the delta. Compared to the existing-condition scenario, modeled vegetation dis- tributions do not change under the existing condition with late-drawdown scenario, a xeric herbaceous community type is greatly expanded under the full-drawdown scenario, and mesic herbaceous, scrub-shrub, and forested community types are greatly expanded under the full-pool scenario. The results of this study are twofold. First, the linked groundwater and vegetation model is relatively simple to construct and can be used to efficiently simulate multiple surface-water and groundwater management scenarios. Second, changes in res- ervoir operations can have pronounced effects on shallow groundwater and associated vegetation distributions in riverine and reservoir-fringe systems. Thus, the effects of chang- ing reservoir operations must be considered if the management of shallow groundwater and associated plant and wildlife habitat resources is to be successful.

Quantifying process-based mitigation strategies in historical context: separating multiple cumulative effects on river meander migration

Environmental legislation in the US (i.e. NEPA) requires defining baseline conditions on current rather than historical ecosystem conditions. For ecosystems with long histories of multiple environmental impacts, this baseline method can subsequently lead to a significantly altered environment; this has been termed a ‘sliding baseline’. In river systems, cumulative effects caused by flow regulation, channel revetment and riparian vegetation removal significantly impact floodplain ecosystems by altering channel dynamics and precluding subsequent ecosystem processes, such as primary succession. To quantify these impacts on floodplain development processes, we used a model of river channel meander migration to illustrate the degree to which flow regulation and riprap impact migration rates, independently and synergistically, on the Sacramento River in California, USA. From pre-dam conditions, the cumulative effect of flow regulation alone on channel migration is a reduction by 38%, and 42–44% with four proposed water diversion project scenarios. In terms of depositional area, the proposed water project would reduce channel migration 51–71 ha in 130 years without current riprap in place, and 17–25 ha with riprap. Our results illustrate the utility of a modeling approach for quantifying cumulative impacts. Model-based quantification of environmental impacts allow scientists to separate cumulative and synergistic effects to analytically define mitigation measures. Additionally, by selecting an ecosystem process that is affected by multiple impacts, it is possible to consider process-based mitigation scenarios, such as the removal of riprap, to allow meander migration and create new floodplains and allow for riparian vegetation recruitment.