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Smaller Streams (smaller + stream)
Selected AbstractsShort-term climatic trends affect the temporal variability of macroinvertebrates in California ,Mediterranean' streamsFRESHWATER BIOLOGY, Issue 12 2007LEAH A. BĘCHE Summary 1. Long-term studies in ecology are essential for understanding natural variability and in interpreting responses to disturbances and human perturbations. We assessed the long-term variability, stability and persistence of macroinvertebrate communities by analysing data from three regions in northern California with a mediterranean-climate. During the study period, precipitation either increased or decreased, and extreme drought events occurred in each region. 2. Temporal trends in precipitation resulted in shifts from ,dry-year' communities, dominated by taxa adapted to no or low flow, to ,wet-year' communities dominated by taxa adapted to high flows. The abundance of chironomid larvae was an important driver of community change. Directional change in community composition occurred at all sites and was correlated with precipitation patterns, with more dramatic change occurring in smaller streams. 3. All communities exhibited high to moderate persistence (defined by the presence/absence of a species) and moderate to low stability (defined by changes in abundance) over the study period. Stability and persistence were correlated with climatic variation (precipitation and El Nińo Southern Oscillation) and stream size. Stability and persistence increased as a result of drought in small streams (first-order) but decreased in larger streams (second- and third-order). Communities from the dry season were less stable than those from the wet-season. 4. This study demonstrates the importance of long-term studies in capturing the effects of and recovery from rare events, such as the prolonged and extreme droughts considered here. [source] Winter streamflow variability, Yukon Territory, CanadaHYDROLOGICAL PROCESSES, Issue 4 2002R. D. Moore Abstract Knowledge of winter streamflow regimes is required in northern catchments to evaluate water supply and to assess the vulnerability of aquatic habitat. The objective of this study was to explore the nature and causes of winter streamflow variability in northern rivers through examination of a limited number of case studies involving intensive field measurements, as well as a synoptic analysis of winter streamflow measurements archived by Water Survey of Canada for rivers in Yukon Territory, Canada. Evidence was found for an abrupt decrease in discharge at freeze-up in one of the case studies and for 10 of the 25 stations in the synoptic analysis that had measurements within 30 days of freeze-up (an additional 12 stations had no measurements within 30 days of freeze-up). However, given the paucity of measurements in the early winter, the magnitude, duration and frequency of these events cannot be specified. The case studies indicate that, even where a coherent depression does not occur, discharge can fluctuate around a smooth recession trend for about the first 30 days after the onset of ice effects, probably as a result of transient storage and release of water behind ice jams. A storage-depletion model that represents streamflow as outflow from two parallel linear reservoirs provided a reasonable fit to most of the observed measurements (excluding those in the first 30 days following freeze-up), with model fit deteriorating with increasing latitude and decreasing catchment size. The effect of latitude could relate to abstraction of flow by ice production, which would cause deviations from a storage-depletion trend. Northern catchments also tended to have steeper late-winter recessions, which could reflect a lack of extensive, deep aquifers to maintain late-winter discharge. The tendency to poorer model fit in smaller catchments could reflect a problem with data reliability, since it is more difficult to find good winter gauging sections in smaller streams. Some evidence for temperature-related discharge fluctuations was found in both the case studies and synoptic analyses. However, the magnitude of these effects appears to be about ±10 to 15%, at most, and not to be consistent between winters. Further advances in understanding winter streamflow variability will require frequent measurements on a range of streams over a number of winters. Copyright © 2002 John Wiley & Sons, Ltd. [source] Bed Stability and Sedimentation Associated With Human Disturbances in Pacific Northwest Streams,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 2 2009Philip R. Kaufmann Abstract:, To evaluate anthropogenic sedimentation in United States (U.S.) Pacific Northwest coastal streams, we applied an index of relative bed stability (LRBS*) to summer low flow survey data collected using the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program field methods in a probability sample of 101 wadeable stream reaches. LRBS* is the log of the ratio of bed surface geometric mean particle diameter (Dgm) to critical diameter (D*cbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. We used a formulation of LRBS* that explicitly accounts for reductions in bed shear stress that result from channel form roughness due to pools and wood. LRBS* ranged from ,1.9 to +0.5 in streams within the lower quartile of human riparian and basin disturbance, and was substantially lower (,4.2 to ,1.1) in streams within the upper quartile of human disturbance. Modeling results suggest that the expected range of LRBS* in streams without human disturbances in this region might be generally between ,0.7 and +0.5 in either sedimentary or volcanic lithology. However, streams draining relatively soft, erodible sedimentary lithology showed greater reductions in LRBS* associated with disturbance than did those having harder, more resistant volcanic (basalt) lithology with similar levels of basin and riparian disturbance. At any given level of disturbance, smaller streams had lower LRBS* than those with larger drainages. In sedimentary lithology (sandstone and siltstone), high-gradient streams had higher LRBS* than did low-gradient streams of the same size and level of human disturbance. High gradient streams in volcanic lithology, in contrast, had lower LRBS* than low-gradient streams of similar size and disturbance. Correlations between Dgm and land disturbance were stronger than those observed between D*cbf and land disturbance. This pattern suggests that land use has augmented sediment supplies and increased streambed fine sediments in the most disturbed streams. However, we also show evidence that some of the apparent reductions in LRBS*, particularly in steep streams draining small volcanic drainages, may have resulted in part from anthropogenic increases in bed shear stress. The synoptic survey methods and designs we use appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. More precise field measurements of channel slope, cross-section geometry, and bed surface particle size would be required to use LRBS* in applications requiring a higher degree of accuracy and precision, such as site-specific assessments at individual streams. [source] Influence of mapping resolution on assessments of stream and streamside conditions: lessons from coastal Oregon, USA,AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 3 2009Ken Vance-Borland Abstract 1.Digital hydrographic data are commonly employed in research, planning, and monitoring for freshwater conservation, but hydrographic datasets differ in spatial resolution and accuracy of spatial representation, possibly leading to inaccurate conclusions or unsuitable policies for streams and streamside areas. 2.To examine and illustrate the potential for different hydrographic datasets to influence in-channel and streamside characterizations, a study area in the US Pacific Northwest was chosen because 1:100,000, 1:24,000, and densified 1:24,000 hydrography are available and widely used in research and management for several species of Pacific salmon and trout at risk. The potential was examined for differences among the digital hydrographic datasets in: (1) spatial extent to influence estimated abundances of fish habitat, streamside buffer conditions, and fish distributions; and (2) spatial position to influence estimated streamside buffer conditions and estimated stream gradient. 3.The analysis of spatial extent found the total stream length represented by the 1:100,000 hydrography was approximately one half that of 1:24,000 hydrography and only one fifth that of densified 1:24,000 hydrography. The 1:100,000 and 1:24,000 networks differed significantly for 13 out of 18 fish habitat attributes, and the three hydrographic datasets differed significantly for many characteristics in streamside buffers; fish distributions mapped at 1:24,000 added 6,14% of stream length to 1:100,000 distributions. The analysis of spatial position found few differences between the 1:100,000 and 1:24,000 hydrography in streamside buffer characteristics but significant differences in channel gradient. 4.Overall, hydrographic datasets differed only slightly in spatial position but differed in spatial extent to the point of representing different populations of streams. If species inhabiting larger streams (greater mean annual discharge) are of interest, then results derived from studies based on 1:100,000 hydrography should prove useful. However, higher-resolution hydrography can be critical when designing and implementing strategies to protect fish and other aquatic species at risk in smaller streams. Copyright © 2008 John Wiley & Sons, Ltd. [source] | |||||||||||||