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the widths featured in the Tuttle Draw valley bottom, although the overall valley bottom <br />• shape in Calamity Draw is less steep and confining as Tuttle Draw. Unconsolidated <br />streaml aid deposits are also commonly found in the bottoms of Calamity Draw. <br />Channel Characteristics. The perennial nature of flow occurrence in both Tuttle and <br />Calamity Draws in the vicinity of mining is largely due to irrigation. Surface water <br />(irrigation return water) and to a lesser extent, ground water (originating from the <br />upland, irrigated fields) discharging into each draw has resulted in deeply incised, <br />narrow main channels that have relatively stable bank sides. Vegetation encroachment over <br />time near and on the channel banks has enabled each channel to stabilize the incised <br />channel geometry and stream course. <br />The deeply incised but yet stable characteristics of the main channels in Calamity and, to <br />a lesser extent, Tuttle Draws, can be attributed to the increased influence by man's <br />activities. In a typical semi-arid environment, ( 15 inches annual precipitation) channel <br />geometries and courses generally reflect the infrequent and highly variable discharges and <br />sediment loads resulting from intense Sumner thunderstorms. These semiarid ephemeral <br />channels often feature shallow and wide bottoms with unstable banksides. Ephemeral <br />• channels will adjust geometries and courses to accommodate the varying discharges and high <br />sediment loads, often radically changing local reach geometries and courses during large, <br />flash flood events. However, as more upland areas are transformed from natural, semi-arid <br />vegetation to pasture, hayland and cropland by irrigation, streamflow variability <br />(discharge and sediment loads) becomes dampened and is controlled by the increased <br />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 <br />loads. This occurrence is reflected in both the main channels of Calamity and Tuttle <br />Draws. <br />Stream gradients have been determined for select reaches along both Tuttle and Calamity <br />Draws, as well as "return ditch" channels tributary to each. Table 7-37 presents stream <br />channel gradients calculated for reaches between select surface water monitoring sites in <br />the vicinity of both the Nucla Mine and the Nucla East mining area, The reader should <br />refer to Exhibit 7-1 for the locations of each referenced monitoring site. From Table <br />7-37, it is evident that the overall stream gradients for both Tuttle and Calamity Draws <br />are similar (.0229 ft/ft and .0248 ft/ft, respectively), However, within each basin, <br />. gradients of certain reaches within each stream channel can vary. For instance, the <br />7-108 Revised 04/11/88 <br />