Rocky Mountain Forest and Range Experiment Station, Fort Collins, ColoradoThe Fool Creek watershed at the Fraser Experimental Forest, Colorado was harvested using a pattern of alternating clearcut and forested strips in 1956. Today, with almost 30 years of postharvest record, subtle impacts on the hydrology of the watershed can be detected that were not significant in the past. In addition to the depositional increases in the snowpack in the openings, average peak water equivalent over the entire watershed has been increased (9%). Long-term, postharvest, climatic records now available show a strong correlation between estimated increases in flow and winter and melt period precipitation. Much of the annual variability in increased flow, now explained by precipitation, was formerly attributed to regrowth or time. Peak discharges, advanced 7.5 days following harvest, have also been increased 20%, with the largest effect occurring in the wettest years. Increases in peak water equivalent, annual flow, and date of peak flow occurrence all appear to be returning to preharvest levels at a very slow rate. Numerous paired watershed experiments have been conducted on the effect of timber harvest on water yield. Regional summaries of these efforts have been well documented byDoubtlass [1983], Harr [1983], Hibbert [1983], Kattelmann et al. [1983], and Troendle [1983], while Bosch and Hewlett [1982] summarized almost 100 experiments worldwide. One watershed experiment, the Fool Creek study [Goodell, 1959; Hoover and Leaf, 1967; Leaf, 1975; Troendle and Leaf, 1981] has become a benchmark for watershed, response in the Rocky Mountain Region. The treatment effect reported is unique worldwide because of its longevity in a semiarid region. This paper is intended to build on those of the past because the everlengthening posttreatment record is allowing more subtle inferences to be drawn on the hydrologic impact of the treatment, as well as what appears to be a very slow recovery to preharvest conditions. Fool Creek Experiment The study began in the early 1940's at the Fraser Experimental Forest in Colorado. The streamgage on Fool Creek, the 289-ha treatment watershed, was built in 1941; the gage on East St. Louis Creek, the 803-ha control watershed, was built in 1943. The paired watersheds were calibrated from 1943 until 1952, at which time the road system was built on Fool Creek. Approximately 14 ha of the watershed were impacted by roads and log decks. After 2 years of postroading stabilization, the watershed was harvested during the summers of 1954, 1955, and 1956. The objective of the experiment was to determine the effect that harvesting has on snowpack accumulation,-•ediment production, and the total yield and timing of streamflow. Forty-percent of the watershed was harvested (50% of the timbered area) using alternating cut and leave strips which varied from 1 to 6 tree heights wide (Figure 1). Snow courses located on both watersheds were monitored about April 1 each year from 1943 to 1954 to calibrate the relationship of peak...
Differences in the transport rate and size of bedload exist for varying levels of flow in coarse-grained channels. For gravel-bed rivers, at least two phases of bedload transport, with notably differing qualities, have been described in the literature. Phase I consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface. Transport rates during Phase II are considerably greater than Phase I and more coarse grains are moved, including material from both the channel surface and subsurface. Transition from Phase I to Phase II indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels. While the existence of different phases of transport is generally acknowledged, the threshold between them is often poorly defined. We present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel-bed channels in Colorado and Wyoming, USA. The piecewise regression recognizes the existence of different linear relationships over different ranges of discharge. The inflection, where the fitted functions intersect, is interpreted as the point of transition from Phase I to Phase II transport; this is termed breakpoint. A comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during Phase I transport, indicating negligible movement of grains in this size range. Gravel larger than about D 16 of the channel surface is more consistently trapped during Phase II transport. The persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached, even though the size of the sediment approaches the size limits of the sampler (76 mm in all cases). A relative breakpoint (R br ) was defined by the ratio between the discharge at the breakpoint and the 1Ð5-year flow (a surrogate for bankfull discharge) expressed as a percentage. The median value of R br was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of R br ranged from about 60 to 100 percent. Variation in this value appears to be independent of drainage area, median grain size, sorting of bed materials, and channel gradient, at least for the range of parameters measured in 12 gravel-bed channels. Published in
Since the early 1990s, US Forest Service researchers have made thousands of bedload measurements in steep, coarse-grained channels in Colorado and Wyoming, USA. In this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes, including step-pool, plane-bed, pool-riffle, and near-braided channels. This effort builds upon previous work where we applied a piecewise regression model to (1) relate flow to rates of bedload transport and (2) define phases of transport in coarse-grained channels. Earlier, the model was tested using bedload data from eight sites on the Fraser Experimental Forest near Fraser, Colorado. The analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied. The earlier results were, however, derived from data collected at sites that, for the most part, have quite similar geology and runoff regimes. In this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions. The results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs. The transition from primarily low rates of sand transport (phase I) to higher rates of sand and coarse gravel transport (phase II) occurs, on average, at about 80 per cent of the bankfull (1·5-year return interval) discharge. A comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase I transport. Moreover, the movement and capture of the D 16 to D 25 grain size of the bed surface seems to correspond with the onset of phase II transport, particularly in systems with largely static channel surfaces. However, while there were many similarities in observed patterns of bedload transport at the 19 studied sites, each had its own 'bedload signal' in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system. Published in 2005 by John Wiley & Sons, Ltd.
ABSTRAn:With the exception of the Sierra-Cascade mountain ranges, the Rocky Mountain chain is the only portion of the western United States that consistently yields more than 3 cm of flow annually.Ten to 15 percent of the land mass in the redon produces the majority of the total flow. This paper addresses the opportunities for increasing flow through forest manipulation, and summarizes the research base that has yielded the current "state of the art" understanding of how snow pack and vegetation management can influence water yield. The optimal harvest design appears to consist of small openings. irregularly shaped, and about 3 to 8 tree heights in width parallel to the wind. (KEY TERMS: watershed manage;ent; water yield; water yield augmentation; snow pack management; vegetation manipulation; hydrology.) 'Paper No. 83077 of the Wuter Resources Bulletin. 'Principal Hydrologist, Rocky Mountain Forest and Range Experiment Station, USDA Forest Service, 240 W. Prospect St.. Fort Collins, Colorado 359 WATER RESOURCES BULLETIN 80526.
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