2013-04-18_PERMIT FILE - C2009087 (6)

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The Yampa River in turn flows into the Green River and finally into the Colorado River. These river systems are explained thoroughly in Iorns et al.'s (1965) USGS Professional Paper 441 -A, • Water Resources of the Upper Colorado River Basin - Technical Report In this report, these river drainages are placed in the Green Division of the Upper Colorado River Basin and in the Yampa River Basin subdivision. Because of the gradual increase in the development of water resources within the Grand Division in the 20th century, streamflow records used for the report (1914 -1957) were adjusted to reflect the level of water -use developments (diversions, dams, irrigation, etc.) that existed in 1957. In spite of the additional activities (diversions, dams, irrigation, etc.) that have occurred within the Yampa River Basin since 1957, the information presented in Iorns et al. (1965) report is considered to be representative of regional hydrologic conditions and should be considered in that context. Regional Surface Water Flow - The higher streamflows within the Yampa River Basin largely result from snowmelt during late spring and early summer (April through June). By late July, most stream discharges recede to baseflow, and remain that way through the following March. Contributions to stream discharges from infrequent summer thunderstorms are minimal. Groundwater discharge from localized aquifers provides baseflow to streams during summer and through the fall. Smaller drainage areas tributary to Grassy Creek and Fish Creek are either intermittent, flowing seasonally, or ephemeral, flowing only briefly in response to snowmelt or intense precipitation. Iorns et al. (1965b) presents flow - duration tables including data for the Yampa River at Steamboat Springs, Colorado. The tables indicate that 90 percent of the time, discharge equals or exceeds 73 cubic feet per second (cfs), and that discharge equals or exceeds 140 cfs 50 percent of the time. The • mean discharge for the Yampa River at this location is 472 cfs. The flow - duration curve constructed from these tables for the Yampa River at Steamboat Springs is typical of snowmelt -type streams. Regional Surface Water Quality - The geologic units and soils over which runoff water flows greatly influence the chemical quality of the surface water. In the Green River division of the Upper Colorado River Basin, and more proximate, within the Yampa River Basin, this statement has been supported by Iorns et al. (1965b). The Yampa River, into which Grassy Creek flows, originates in the Park Range Mountains which are principally composed of igneous (Tertiary volcanics) and metamorphic (Precambrian) rocks. The lower - altitude, interior portions of this basin are chiefly underlain by marine and continental rocks (Lewis Shale, Williams Fork Formation, Iles Formation, Mancos Shale) and predominantly continental rocks (Dakota Sandstone, Morrison Formation). The marine and continental rocks and the predominantly continental rocks have the greatest potential for influencing water chemistry through solution of readily available minerals. The upper three of the marine /continental formations (Lewis, Williams Fork, and Iles; as represented by the Lewis Shale, Twentymile Sandstone, Wadge Coal, and Trout Creek Sandstone) outcrop within the PSCM permit area (Map 2.04.5 -M1, Regional Geology and Hydrology). According to data presented in Iorns et al. (1965b), at times of high flow generated largely by snowmelt runoff from the upland areas (underlain by igneous and metamorphic rocks), the water is of the calcium bicarbonate type. During low flow, the water features higher percentages of sodium, magnesium, and sulfate. As flow progresses downstream along the Yampa River, the water type • generally remains dominated by calcium or sodium, and bicarbonate. PSCM Permit App. 2.04 -20 6/15/09