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Many of the changes in stability are associated with changes in the magnitude, duration and timing of streamflows. Research conducted by the USDA Forest Service has demonstrated significant, long-term increases in streamflow following timber harvest and road construction. These changes in flow are associated with reductions in forest evapotranspiration, rainfall interception loss, an increase in snowpack water equivalent and other processes, making excess water available for runoff. A snowmelt hydrograph for Fool Creek, in the Central Rockies Province of Colorado shows changes in peak and duration of flows following a 50 per cent stand removal by patch clearcutting on a second order stream (Figure 50) (Troendle and Olsen 1993).
Figure 50. Water yield increase following patch clearcutting - Fool Creek, Colorado (from Troendle and Olsen 1993).
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Streamflow increases can potentially alter the sediment transport relations and morphological character of rivers. The response of the stream to increased streamflow is a function of both the stream type and the stability of the channel, as well as the magnitude and duration of flow changes. Numerous studies have demonstrated that forest disturbances can increase the amount of introduced sediment to the channel. Bosch and Hewlett (1982) summarized the effect of timber harvest on increased streamflow for over 100 experiments worldwide. Following partial clearcutting on Deadhorse Creek Colorado, a significant increase in both sediment export and flow was observed (Troendle and Olsen 1993). The analysis of Deadhorse Creek indicated that the increase in sediment was from within the channel and not the result of increased sediment introduction following road building and harvest. The timber harvest and roads shown in the Willow Creek watershed, Colorado (Figure 51) depict high potential for increased streamflow and corresponding sediment yields due to the extent and nature of the forest disturbance. Flow-related increases in sediment, however, have not been widely reported. Cumulative effects of such practices need to be assessed for their respective influence on increased sediment yields and channel instability.
Figure 51. Cumulative effects of clearcutting and road construction - Willow Creek drainage, Colorado.
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Water yield models for both snowmelt and stormflow-generated runoff have been developed in use by hydrologists for many years. The snowmelt model in WRENSS (USEPA 1980) was validated nationwide and is recommended in WARSSS for snowmelt-dominated regions. The Unit Hydrograph and flow duration curve methods such as TR 20 and TR 55, WRENSS, (USEPA 1980) and recently described in Haan et al. (1994), are models that predict a runoff response based on a given storm. These approaches are applied in urban watersheds and for stormflow-dominated regions. The snowmelt version by Sheppard et al. (1991) is presently being updated for a windows-based program by R. Swanson.
Diversions also affect flow and sediment dynamics. Flow increases due to imported water can create excessive shear stress and stream power in channels leading to enlargement, degradation and excessive bank erosion. Decreases in flow or dewatering of mainstem channels can reduce the available energy of the channel to maintain transport capacity, thus the sediment delivered to the regulated trunk stream by unregulated tributaries leads to aggradation. Flow releases by reservoirs and/or diversions can be determined by the operational hydrology of the system or back-calculated from high stage scour lines in the channel using indirect methods of discharge.
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