1988-04-11_PERMIT FILE - C1981008A

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the widths featured in the Tuttle Draw valley bottom, although the overall valley bottom shape in Calamity Draw is less steep and confining as Tuttle Draw. Unconsolidated streamlaid deposits are also commonly found in the bottoms of Calamity Draw. Channel Characteristics. The perennial nature of flow occurrence in both Tuttle and Calamity Draws in the vicinity of 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 each 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 each channel to stabilize the incised channel geometry and stream course. The deeply incised but yet stable characteristics of the main channels in Calamity and, to a lesser extent, Tuttle Draws, 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 sediment loads, often radically changing local reach geometries and courses during large, flash flood events. However, as more upland areas are transformed from natural, semiarid vegetation to pasture, hayland and cropland by irrigation, streamflow variability (discharge and sediment loads) becomes dampened and is controlled by the increased vegetation and manipulation of the 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 and Tuttle Draws. Stream gradients have been determined for select reaches along both Tuttle and Calamity Draws, as well as "return ditch" channels tributary to each. Table 7-37 presents stream channel gradients calculated for reaches between select surface water monitoring sites in the vicinity of both the Nucla Mine and the Nucla East mining area. The reader should refer to Exhibit 7-1 for the locations of each referenced monitoring site. From Table 7-37, it is evident that the overall stream gradients for both Tuttle and Calamity Draws are similar (.0229 ft/ft and .0248 ft/ft, respectively). However, within each basin, gradients of certain reaches within each stream channel can vary. For instance, the 7-108 Revised 04/11/88