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Channel Characteristics <br />The perennial nature of flow occurrence in both Tuttle Draw and Calamity Draw in the vicinity of <br />mining is largely due to irrigation. Surface water (irrigation return water) and to a lesser extent, <br />ground water (originating from the upland, irrigated fields) discharging into Calamity Draw has <br />resulted in deeply incised, narrow main channels that have relatively stable bank sides. Vegetation <br />encroachment over time near and on the channel banks has enabled the channel to stabilize the <br />incised channel geometry and stream course. In contrast, Tuttle Draw is developed on resistant <br />bedrock. <br />The deeply incised but yet stable characteristics of the main channel(s) in Calamity and to a lesser <br />extent Tuttle Draw can be attributed to the increased influence by man's activities. In a typical <br />semi-arid environment, (15 inches annual precipitation) channel geometries and courses generally <br />reflect the infrequent and highly variable discharges and sediment loads resulting from intense <br />summer thunderstorms. These semi-arid ephemeral channels often feature shallow and wide <br />bottoms with unstable banksides. Ephemeral channels will adjust geometries and courses to <br />accommodate the varying discharges and high sediment loads, often radically changing local reach <br />geometries and courses during large, flash flood events. However, as more upland areas are <br />transformed from natural, semi-arid vegetation to pasture, hayland and cropland by irrigation, <br />streamflow variability (discharge and sediment loads) becomes dampened and is controlled by the <br />increased vegetation and manipulation of the drainage system. This results in channels becoming <br />more stable and often more incised, due to lower, less variable flows with lesser sediment loads. <br />This occurrence is reflected in both the main channels of Calamity Draw and Tuttle Draw. <br />Gradients of certain reaches within the stream channel varies. For instance, the overall stream <br />gradient of Calamity Draw is .0248 ft/ft while the segment of the reach of Calamity Draw nearest <br />the New Horizon 2 mining area (SW-N101 to SW-N107) is .0083 ft/ft. Gradients calculated forthe <br />monitored tributaries to the main channel have gradients from 4 up to 16 times as great as the <br />main channel reach. The gradient for the reach adjacent to and north of the permit area for Tuttle <br />Draw is 0.0015 ft/ft, (see Table 7-37 of Volume B New Horizon 1 Reclamation Plan). <br />(Revised 25 Jul 2002) 2.04.7-25 <br />