We compared stream ecosystem responses to two types of disturbances: flood and debris flows. A large storm in February 1996 disturbed four similarly sized sub‐watersheds of the Calapooia River, in the Cascade Mountains of Oregon, USA. All sub‐watersheds had flood and in two, 31 and 81% of the perennial channel, a debris flow affected the channel. For 8 years, we used a suite of approaches: stream temperature, nutrient regime, periphyton and macroinvertebrate assemblages, and resident trout abundance and habitat to evaluate the persistence of instream impacts. Differences in stream temperatures and nitrate‐nitrogen concentrations were evident, probably, due to the removal of vegetation and modification of riparian soils at the debris flow sites. Instream biological responses varied. After the event, fish, including trout, were rare with no fish at the debris flow sites. Within 6 years, trout densities (Oncorhynchus mykiss and Oncorhynchus clarki) were similar and young‐of‐the‐year trouts were common. In contrast, periphyton and macroinvertebrate assemblages differed. Periphyton biomass was lower and nitrogen‐fixing periphyton was more abundant at the debris flow sites. Macroinvertebrate assemblage diversity was higher at the debris flow sites due to fewer dominant taxa. Macroinvertebrate functional feeding groups also differed with fewer gatherers and more scrapers at the debris flow sites. Debris flow impacts related to loss of riparian canopy will probably persist until mature red alder stands are re‐established along stream‐reaches affected by debris flows to provide nitrogen input and shade. Copyright © 2011 John Wiley & Sons, Ltd.
Drifting invertebrates from small headwater streams are a food subsidy for fi shes and other downstream consumers in larger streams. This subsidy can become especially important for stream fi shes during summer due to high metabolic demand caused by elevated water temperatures. The source length in small streams contributing these drifting invertebrates to fi sh bearing habitat has not been established. We conducted an experiment to determine if summer drift delivered by headwater streams to larger channels is infl uenced by contributions originating more than 100 m upstream. We conducted this experiment on tributaries of the Calapooia River, Oregon, USA. We blocked drift with 250 µm mesh nets in three streams 100 m upstream of our sample nets continuously for 48 d. We also sampled fi ve streams without blocking nets. Four 24 h samples of drift were collected prior to the placement of blocking nets and every two weeks afterwards at each stream. Overall mean abundance and mean biomass of drift did not differ between blocked and unblocked streams for any sampling period. There was a temporal trend at all sites of declining invertebrate abundance and biomass. We found no treatment effect for any of the most common insect orders or for the 16 genera that comprised at least 1 % of the total organisms captured. The majority of invertebrate drift is of local origin in headwater streams during summer basefl ow.
The fall Chinook Salmon (Oncorhynchus tshawytscha) population that spawns in the Hanford Reach of the Columbia River, USA is paradoxical because it is located above 4 and below 10 main stem Columbia River dams and yet is one of the largest and most productive C. salmon populations in the Pacific Northwest. A synthesis of information collected in the Hanford Reach reveals that the hydrosystem above the Hanford Reach and the management of river flows may have contributed to the recent size and productivity of this salmon population. Mechanisms for high survival and capacity at each freshwater life stage have been identified. Plausible mechanisms for contributing to high spawning capacity include: (a) more spawning habitat available during the spawning period. Plausible mechanisms for high egg‐to‐presmolt survival include: (a) reduced desiccation of redds, (b) reduced scour of redds, (c) reduced sedimentation in redds, (d) improved flow exchange within redds, and (e) increased food availability. Smolt survival may also be enhanced through the large size they attain in the Hanford Reach. This synthesis of information provides an uncommon assessment of some the positive effects of flow management from hydropower dams on a valued native fish that has occurred over the last three decades. WIREs Water 2018, 5:e1275. doi: 10.1002/wat2.1275 This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems Water and Life > Conservation, Management, and Awareness
We examined the structure of periphyton and macroinvertebrate assemblages in 18 Oregon Coast Range perennial headwater streams bordered by mature (6), clearcut (5), and thinned (7) forest treatments. Summer base flow discharge ranged from 0.02 to1.98 l/s (mean − 0.5 l/s). Periphyton was almost entirely diatoms (72 species) and assemblage characteristics were most strongly influenced by substrate characteristics. Mature treatment sites had fewer species of diatoms and less biomass than other treatments. Diatom richness was highest at sites with higher unit area discharge. Diatom assemblages were dominated by a single species (Achnanthes lanceolata [Brébisson] Grunow), which composed more than 50% relative abundance of the assemblage at nine sites. Macroinvertebrate assemblages were rich, with 194 taxa collected across all sites and 42 taxa found at a single site. Macroinvertebrate assemblages differed across treatments with higher abundance, more Chironomidae taxa, and more biomass at the clearcut treatment sites. We observed no difference in functional feeding group percentage composition across treatments, with collector-gatherers and shredders composing at least 50% abundance at all treatments. We found little difference in either periphyton or macroinvertebrate assemblages between thinned and mature treatments.
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