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mining is largely due to irrigation. Surface water (irrigation return water) and to a lesser extent, ground water (originating from the upland, irrigated fields) discharging into Calamity Draw has resulted in deeply incised, narrow main channels that have relatively stable bank sides. Vegetation encroachment over time near and on the channel banks has enabled the channel to stabilize the incised channel geometry and stream course. In contrast, Tuttle Draw is developed on resistant bedrock. The deeply incised butyet stable characteristics ofthemain channel(s) in Calamity and to a lesser extent Tuttle Draw can be attributed to the increased influence by man's activities. In a typical semi-arid environment, (15 inches annual precipitation) channel geometries and courses generally reflect the infrequent and highly variable discharges and sediment loads resulting from intense summer thunderstorms. These semi-arid ephemeral channels often feature shallow and wide bottoms with unstable banksides. Ephemeral channels will adjust geometries and courses to accommodate the varying discharges and high sedimentloads, often radically changing local reach geometries and courses during large, flash flood events. However, as more upland areas are transformed from natural, semi-arid vegetation to pasture, hayland and cropland by irrigation, • streamflow variability (discharge and sediment loads) becomesdampened and is controlled by the increased vegetation and manipulation ofthe drainage system. This results in channels becoming more stable and often more incised, due to lower, less variable flows with lesser sediment loads. This occurrence is reflected in both the main channels of Calamity Draw and Tuttle Draw. Gradients of certain reaches within the stream channel varies. For instance, the overall stream g radient of Calamity Draw is .0248 fUft while the segment of the reach of Calamity Draw nearest the New Horizon 2 mining area (SW-N101 to SW-N107) is .0083 ft/ft. Gradients calculated for the monitored tributaries to the main channel have gradients from 4 up to 16 times as great as the main channel reach. The gradient for the reach adjacent to and north of the permit area for Tuttle Draw is 0.0015 ft/ft, (see Table 7-37 of Volume B New Horizon 1 Reclamation Plan). Predicted Peak Flows The annual mean and peak discharges were predicted for both Tuttle Draw and Calamity Draw using techniques outlined in the USGS Water Resources Investigations Report 85-4086 titled Estimation of Natural streamflow Characteristics in Western Colorado (Kircher et al. 1985). The • (REVISED6/28/01) 2.04.7-24