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Long-term flow records from sub-watersheds in the Caspar Creek Experimental Watersheds were used to test the accuracy of four methods commonly used to estimate peak flows in small forested watersheds: the Rational Method, the updated USGS Magnitude and Frequency Method, flow transference methods, and the NRCS curve number method. Comparison of measured and calculated results for 10-year return-interval flows demonstrates that, under the conditions tested, the direct flow transference method provides the most reliable results if suitable data are available; results for 100-year flows show similar patterns. None of the methods consistently underestimated the values derived from the gaging record. This indicates that these methods are unlikely to result in an under-design of drainage structures with respect to flow capacity. However, design of stable stream crossings in steep forested areas also requires consideration for passage of sediment, woody debris, and fish, so estimation of required flow capacity represents only a first step in the design process. [ABSTRACT FROM AUTHOR]

Large landslides can exert a lasting influence on hillslope and channel form and can continue to contribute to high in-stream sediment loads long after the event. We used discharge and suspended sediment concentration data from the Caspar Creek Experimental Watersheds to evaluate the temporal distribution of sediment inputs from 11 landslides of 100 to 5500 m3. Slide-related suspended sediment loads were estimated as deviations from expected loads referenced to nearby control watersheds. For the two largest slides, suspended sediment export during the year of the slide accounted for 5 and 15 percent of the initial slide volumes, while subsequent export accounted for an additional 8 and 2 percent over the period for which export has been tracked (8 and 10 years). Regressions of excess sediment against time and storm size indicate that suspended sediment loads are likely to recover more quickly for small storms than large ones. Measurements of sediment storage along channels affected by the slides generally showed aggradation for 1 to 2 years. For the largest slide, however, downstream accumulation has now continued for at least 8 years. Nearly half of the sediment initially displaced by that slide remains in storage adjacent to channels and so may be subject to re-mobilization during future storms. In addition, in-channel deposits have triggered bank erosion and diverted the channel in places; much of the downstream increase in suspended sediment load during the post-slide years is derived from these secondary sources rather than directly from the slide debris. [ABSTRACT FROM AUTHOR]