fluvial processes

Stream Temperature Relationships to Forest Harvest in Western Washington

Source: 
Journal of the American Water Resources Association
Volume: 
45-1
Year: 
2009
Abstract: 

We compared summer stream temperature patterns in 40 small forested watersheds in the Hoh and Clearwater basins in the western Olympic Peninsula, Washington, to examine correlations between previous riparian and basin-wide timber harvest activity and stream temperatures. Seven watersheds were unharvested, while the remaining 33 had between 25% and 100% of the total basin harvested, mostly within the last 40 years. Mean daily maximum temperatures were significantly different between the harvested and unharvested basins, averaging 14.5C and 12.1C, respectively. Diurnal fluctuations between harvested and unharvested basins were also significantly different, averaging 1.7C and 0.9C, respectively. Total basin harvest was correlated with average daily maximum temperature (r2 = 0.39), as was total riparian harvest (r2 = 0.32). The amount of recently clear-cut riparian forest (<20 year) within 600 m upstream of our monitoring sites ranged from 0% to 100% and was not correlated to increased stream temperatures. We used Akaike’s Information Criteria (AIC) analysis to assess whether other physical variables could explain some of the observed variation in stream temperature. We found that variables related to elevation, slope, aspect, and geology explain between 5% and 14% more of the variability relative to the variability explained by percent of basin harvested (BasHarv), and that the BasHarv was consistently a better predictor than the amount of riparian forest harvested. While the BasHarv is in all of the models that perform well, the AIC analysis shows that there are many models with two variables that perform about the same and therefore it would be difficult to choose one as the best model.We conclude that adding additional variables to the model does not change the basic findings that there is a relatively strong relationship between maximum daily stream temperatures and the total amount of harvest in a basin, and strong, but slightly weaker relationship between maximum daily stream temperatures and the total riparian harvest in a basin. Seventeen of the 40 streams exceeded the Washington State Department of Ecology’s (DOE) temperature criterion for waters defined as ‘‘core salmon and trout habitat’’ (class AA waters). The DOE temperature criterion for class AA waters is any seven-day average of daily maximum temperatures in excess of 16C. The probability of a stream exceeding the water quality standard increased with timber harvest activity. All unharvested sites and five of six sites that had 25-50% harvest met DOEs water quality standard.In contrast, only nine of eighteen sites with 50-75% harvest and two of nine sites with >75% harvest met DOEs water quality standard. Many streams with extensive canopy closure, as estimated by the age of riparian trees, still had higher temperatures and greater diurnal fluctuations than the unharvested basins. This suggests that the impact of past forest harvest activities on stream temperatures cannot be entirely mitigated through the reestablishment of riparian buffers.

Author(s): 

Michael M. Pollock, Timothy J. Beechie, Martin Liermann, and Richard E. Bigley

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Influence of Small Dams on Downstream Channel Characterstics in Pennsylvania and Maryland

Source: 
Journal of the American Water Resource Association
Volume: 
45-1
Year: 
2009
Abstract: 

We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi-quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrockor boulders on the streambed.The median grain diameter (D50) is increased slightly by dam construction,but D50 remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of thedam has therefore only influenced the fraction of finer-grained sediment on the bed, and has not caused other measurable changes in fluvial morphology.The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well-vegetated and cohesive banks, and low rates of bed material supply and transport.If the dams of our study are removed, we argue that long-term changes (those that remain after a period of transient adjustment)will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal instreams similar to those of our study area should not result in significant long-term geomorphic changes.

Author(s): 

Katherine Skalak, James Pizzuto, and David D. Hart

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