Arkansas Basin_Water Supply and Needs

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Section 3 Physical Environment of the Arkansas Basin Table 3.2 Summary of Selected USGS Stream Gages for the Arkansas River Basin Mean Annual Streamflow (cfs) Period of Record (Years) Arkansas at Canon City Fountain Creek at Pueblo Arkansas at Las Animas Purgatoire near Las Animas Arkansas at Lamar Source: USGS National Water Information System (NWIS) web/HydroBase database To monitor streamflow, numerous USGS streamflow gages are maintained in the Arkansas Basin. Five of these gages were selected to summarize historic flows in the basin across a broad spatial scale. The locations of these gages are shown in Figure 3-5. Table 3-2 summarizes the mean annual streamflow, period of record, and drainage area for each gage. As indicated by the table, mean annual flows are highest in the upstream reaches of the Arkansas River near Canon City. Major surface water diversions and segments with decreed instream flow rights are also indicated in Figure 3-5. 3.2.7 Groundwater Groundwater in the Arkansas Basin is located within the following aquifers: . Alluvial Aquifer . Denver Basin . High Plains . Raton Basin . Dakota-Cheyenne . Wet Mountain Valley and Huerfano Figure 3-6 shows the outline of the aquifers broken down into three groups: alluvial, bedrock (Raton Basin and Dakota-Cheyenne), and Designated Basin (High Plains). Also shown in the figure is the location of wells in the Arkansas Basin with a permitted or decreed yield of 500 gallons per minute (gpm) or higher. Information from the 2003 Colorado Ground Water Atlas was used as the basis for this section (CGS 2003). The unconfined alluvial aquifer of the Arkansas River, comprised of glacial silts to large boulders, is primarily recharged by surface water infiltration from the river as well as from many ditches and canals. Irrigation also plays a role in the recharge of the alluvial aquifer. Depth of water in the lower valley generally ranges between 5 o :\SHAWN\ARKANSAS\S3 _ARKANSAS. DOC 738 101 218 93 188 1890-2002 1922-2002 1939-2002 1922-2002 1913-2002 and 30 feet and in the upper valley between 5 and 58 feet. Trends in hydrographs since the 1970s show a general increase in the water table elevation, which can be attributed to irrigation return flows. Irrigation is the major use of the alluvial aquifer groundwater. However, in Chaffee and Lake Counties, public water supply is the primary use of alluvial groundwater. The major aquifers of the Raton Basin include the Raton, Vermejo, and Trinidad formations, and the Cuchara and Poison Canyon formations. Sources of recharge for the aquifers include runoff from the Sangre de Cristo Mountains, precipitation infiltration, and infiltration from streams and lakes. The depth to water generally increases in the aquifers from northwest to southeast, indicating a southeastern direction of groundwater flow. In all areas but the southeast corner of the basin, water can be encountered at less than 200 feet below ground surface. The Dakota-Cheyenne aquifer lies under the majority of the Arkansas Basin. The stratigraphy of this unit ranges from well-sorted sandstone to fine-grained shales. The aquifer provides water for irrigation and domestic water supply in the basin. Due to the diversity of the aquifer stratigraphy, well yields can range from around 5 gpm to over 1,000 gpm. The High Plains aquifer is found in the eastern portion of the basin and is considered a "Designated Basin" by the State of Colorado. A Designated groundwater basin is not adjacent to a continuously flowing natural stream or a stream that fulfills a surface water right. A designated groundwater basin is established by the Colorado Groundwater Commission in accordance with Section 37 -90-106 of the Colorado Revised Statutes. The High Plains aquifer is a major source of water for southeast Colorado. Because of this, groundwater withdrawals have exceeded recharge since the early 1960s. The CDIVI 3-3