Because dams influence the two primary factors-water and sediment-that determine the shape, size, and overall morphology of a river, they represent fundamental interventions in the fluvial system. Decades of research on effects of dams on rivers have yielded abundant case studies of down

Abstract:Stream temperature is a complex function of energy inputs including solar radiation and latent and sensible heat transfer. In streams where groundwater inputs are significant, energy input through advection can also be an important control on stream temperature. For an individual stream reach, models of stream temperature can take advantage of direct measurement or estimation of these energy inputs for a given river channel environment. Understanding spatial patterns of stream temperature at a landscape scale requires predicting how this environment varies through space, and under different atmospheric conditions. At the landscape scale, air temperature is often used as a surrogate for the dominant controls on stream temperature. In this study we show that, in regions where groundwater inputs are key controls and the degree of groundwater input varies in space, air temperature alone is unlikely to explain within-landscape stream temperature patterns. We illustrate how a geologic template can offer insight into landscape-scale patterns of stream temperature and its predictability from air temperature relationships. We focus on variation in stream temperature within headwater streams within the McKenzie River basin in western Oregon. In this region, as in other areas of the Pacific Northwest, fish sensitivity to summer stream temperatures continues to be a pressing environmental issue. We show that, within the McKenzie, streams which are sourced from deeper groundwater reservoirs versus shallow subsurface flow systems have distinct summer temperature regimes. Groundwater streams are colder, less variable and less sensitive to air temperature variation. We use these results from the western Oregon Cascade hydroclimatic regime to illustrate a conceptual framework for developing regional-scale indicators of stream temperature variation that considers the underlying geologic controls on spatial variation, and the relative roles played by energy and water inputs.

Young basalt terrains offer an exceptional opportunity to study landscape and hydrologic evolution through time, since the age of the landscape itself can be determined by dating lava fl ows. These constructional terrains are also highly permeable, allowing one to examine timescales and process of geomorphic evolution as they relate to the partitioning of hydrologic fl owpaths between surface and sub-surface fl ow. The western slopes of the Cascade Range in Oregon, USA are composed of a thick sequence of lava fl ows ranging from Holocene to Oligocene in age, and the landscape receives abundant precipitation of between 2000 and 3500 mm per year. On Holocene and late Pleistocene lava landscapes, groundwater systems transmit most of the recharge to large springs (≥0·85 m 3 s −1) with very steady hydrographs. In watersheds >1 million years old, springs are absent, and well-developed drainage networks fed by shallow subsurface stormfl ow produce fl ashy hydrographs. Drainage density slowly increases with time in this basalt landscape, requiring a million years to double in density. Progressive hillslope steepening and fl uvial incision also occur on this timescale. Springs and groundwater-fed streams transport little sediment and hence are largely ineffective in incising river valleys, so fl uvial landscape dissection appears to occur only after springs are replaced by shallow subsurface stormfl ow as the dominant streamfl ow generation mechanism. It is proposed that landscape evolution in basalt terrains is constrained by the time required for permeability to be reduced suffi ciently for surface fl ow to replace groundwater fl ow.

A key challenge for resource and land managers is predicting the consequences of climate warming on streamflow and water resources. During the last century in the western United States, significant reductions in snowpack and earlier snowmelt have led to an increase in the fraction of annual streamflow during winter and a decline in the summer. Previous work has identified elevation as it relates to snowpack dynamics as the primary control on streamflow sensitivity to warming. But along with changes in the timing of snowpack accumulation and melt, summer streamflows are also sensitive to intrinsic, geologically mediated differences in the efficiency of landscapes in transforming recharge (either as rain or snow) into discharge; we term this latter factor drainage efficiency. Here we explore the conjunction of drainage efficiency and snowpack dynamics in interpreting retrospective trends in summer streamflow during 1950–2010 using daily streamflow from 81 watersheds across the western United States. The recession constant (k) and fraction of precipitation falling as snow (Sf) were used as metrics of deep groundwater and overall precipitation regime (rain and/or snow), respectively. This conjunctive analysis indicates that summer streamflows in watersheds that drain slowly from deep groundwater and receive precipitation as snow are most sensitive to climate warming. During the spring, however, watersheds that drain rapidly and receive precipitation as snow are most sensitive to climate warming. Our results indicate that not all trends in western United States are associated with changes in snowpack dynamics; we observe declining streamflow in late fall and winter in rain‐dominated watersheds as well. These empirical findings support both theory and hydrologic modelling and have implications for how streamflow sensitivity to warming is interpreted across broad regions. Copyright © 2012 John Wiley & Sons, Ltd.

[1] While the impacts of long-term climate change trends on glacier hydrology have received much attention, little has been done to quantify direct glacier runoff contributions to streamflow. This paper presents an approach for determining glacier runoff contributions to streamflow and estimating the effects of increased temperature and decreased glacier area on future runoff. We focus on late summer streamflow (when flow is lowest and nonglacier contributions to flow are minimal) of a small glacierized watershed on the flanks of Mount Hood, Oregon, United States. Field and lab measurements and satellite imagery were used in conjunction with a temperature-index model of glacier runoff to simulate potential effects of increased temperature and reduction in glacier area on late summer runoff in the watershed. Discharge and stable isotope data show that 41-73% of late summer streamflow is presently derived directly from glacier melt. Model simulations indicate that while increased temperature leads to rapid glacier melt and therefore increased streamflow, the consequences of glacier recession overcomes this effect, ultimately leading to substantial declines in streamflow. Model sensitivity analyses show that simulation results are most sensitive to degree day factor and less sensitive to uncertainties in debris-covered area and accumulation area ratio. This case study demonstrates that the effects of glacier recession on streamflow are a concern for water resource management at the local scale. This approach could also be extended to larger scales such as the upper Columbia River basin where glacier contributions to late summer flows are also thought to be substantial.

Abstract:Climate models project warmer temperatures for the north-west USA, which will result in reduced snowpacks and decreased summer streamflow. This paper examines how groundwater, snowmelt, and regional climate patterns control discharge at multiple time scales, using historical records from two watersheds with contrasting geological properties and drainage efficiencies. In the groundwater-dominated watershed, aquifer storage and the associated slow summer recession are responsible for sustaining discharge even when the seasonal or annual water balance is negative, while in the runoff-dominated watershed subsurface storage is exhausted every summer. There is a significant 1 year cross-correlation between precipitation and discharge in the groundwater-dominated watershed (r D 0Ð52), but climatic factors override geology in controlling the interannual variability of streamflow. Warmer winters and earlier snowmelt over the past 60 years have shifted the hydrograph, resulting in summer recessions lasting 17 days longer, August discharges declining 15%, and autumn minimum discharges declining 11%. The slow recession of groundwater-dominated streams makes them more sensitive than runoff-dominated streams to changes in snowmelt amount and timing.

ABSTRACT:In the western United States, climate warming poses a unique threat to water and snow hydrology because much of the snowpack accumulates at temperatures near 0 ∘ C. As the climate continues to warm, much of the region's precipitation is expected to switch from snow to rain, causing flashier hydrographs, earlier inflow to reservoirs, and reduced spring and summer snowpack. This study investigates historical variability in snow to precipitation proportion (S f ) and maps areas in the western United States that have demonstrated higher S f sensitivity to warming in the past. Projected changes in S f under 1.1, 1.8, and 3.0 ∘ C future warming scenarios are presented in relation to historical variability and sensitivity. Our findings suggest that S f in this region has primarily varied based on winter temperature rather than precipitation. The difference in S f between cold and warm winters at low-and mid-elevations during 1916-2003 ranged from 31% in the Pacific Northwest to 40% in the California Sierra Nevada. In contrast, the difference in S f between wet and dry winters was statistically not significant. Overall, in the northern Sierra, Klamath, and western slopes of the Cascade Mountains Ranges, S f was most sensitive to temperature where winter temperature ranged between −5 to 5 ∘ C. Results from our trend analysis show a regional shift in both

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