Laserfiche WebLink
98 feet/mile. <br />Channel Characteristics <br />The perennial nature of flow occurrence in both Tuttle Draw and Calamity Draw in the vicinity of mining is <br />largely due to irrigation. Surface water (irrigation return water) and to a lesser extent, ground water <br />(originating from the upland, irrigated fields) discharging into Calamity Draw has resulted in deeply incised, <br />narrow main channels that have relatively stable bank sides. Vegetation encroachment over time near and <br />on the channel banks has enabled the channel to stabilize the incised channel geometry and stream course. <br />In contrast, Tuttle Draw is developed on resistant bedrock. <br />The deeply incised but yet stable characteristics of the main channel(s) in Calamity and to a lesser extent <br />Tuttle Draw can be attributed to the increased influence by man's activities. In a typical semi -arid <br />environment, (15 inches annual precipitation) channel geometries and courses generally reflect the <br />infrequent and highly variable discharges and sediment loads resulting from intense summer thunderstorms. <br />These semi -arid ephemeral channels often feature shallow and wide bottoms with unstable banksides. <br />Ephemeral 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, flash flood events. <br />However, as more upland areas are transformed from natural, semi -arid vegetation to pasture, hayland and <br />cropland by irrigation, streamflow variability (discharge and sediment loads) becomes dampened and is <br />controlled by the increased vegetation and manipulation of the drainage system. This results in channels <br />becoming 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 gradient of <br />Calamity Draw is .0248 ft/ft while the segment of the reach of Calamity Draw nearest the New Horizon 2 <br />mining area (SW -N101 to SW -N107) is .0083 ft/ft. Gradients calculated for the monitored tributaries to the <br />main channel have gradients from 4 up to 16 times as great as the main channel reach. The gradient for the <br />reach adjacent to and north of the permit area for Tuttle Draw is 0.0015 ft/ft, (see Table 7-37 of Volume B <br />New Horizon I Reclamation Plan). <br />Predicted Peak Flows <br />The annual mean and peak discharges were predicted for both Tuttle Draw and Calamity Draw using <br />techniques outlined in the USGS Water Resources Investigations Report 854086 titled Estimation of <br />Natural Streamflow Characteristics in Western Colorado (Kircher et al. 1985). The techniques used to <br />predict discharge characteristics of Tuttle Draw and Calamity Draw involved the use of regression <br />relationships determined from historical streamflow records in Western Colorado. Basin parameters and <br />precipitation records were used to correlate discharge characteristics of four different regions of Western <br />Colorado: 1) Mountain; 2) Rio Grande: 3) Southwest and; 4) Northwest. <br />Using the regression model for the southwestern region, the mean annual discharge and peak discharges <br />from the 2-, 5-, 20-, 25-, 50-, 100- and 500 -year recurrence intervals were predicted at two locations each in <br />both Tuttle and Calamity draw. For both draws, discharge characteristics were calculated for the entire <br />drainage (at the mouths), and at the most downstream surface water monitoring site in each Draw (SW -N3, <br />Tuttle Draw and SW -N103, Calamity Draw, see Map 2.04.7-1A). See also Table 7-38, Volume B of New <br />Horizon 1 Reclamation Plan. <br />Page 2.04.7-18 March 2017 JR -77) <br />