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7.0 GROUND WATER--Continued 7.2 Depth to Ground Water Ground Water is Less Than 500 Feet Deep Topography and geology control ground-waterdepths. The depth to ground water depends mostly on the topographic position of the well. In stream valleys, ground water is less than Il)D feet deep because the valleys intercept ground water Wowing through the bedrock (fig. 7.2-I). Some of this water discha[ges as springs or (lows into stream alluvium. The alluvium also stores and transmits water infil- trating from precipitation and storm and snowmelt runoff. In Area 61, the alluvium is saturated to as much as 20 feet above [he bedrock near the streams and much less on the edges of valleys. Springs discharge from the alluvium where the underlying bedrock is near the surface or where dikes and sills cross the channels. On stream divides, permeable formations are drained by seeps and springs dis- charging into the valleys, and wells may have to be drilled as much as 500 feet deep to obtain water. In figure 7.2-2, which shows the depth to ground water in a part of the area for which data are available, areas of shallow ground water generally coincide with valleys, and areas o(deep ground water general- lycoincide with stream divides. The depth to ground water also is affected by the geology. An extensive area of shallow ground walcr north of the Apislrapa River (fig. 7.2-2) coincides with outcrop areas of the very permeable Cuchara- Poison Canyon aquifer (fig. 7.2-3). Clusters of springs shown in figure 7.2-3 indicate other areas where ground water is relatively shallow. Many of the springs shown in figure 7.2-3 are located at or near the contact between the Cuchara-Poison Can- yon artd Raton-Vermejo-Trinidad aquifers; most others are located on the slopes of volcanic-capped mesas, along dikes and sills, and in the axial region of the La Veta Syncline. The contact between the Cuchara-Poison Canyon and Raton-Vermejo- Trinidad aquifers localizes spring flow because water descending through loosely consolidated sandstone of the Cuchara-Poison Canyon aquifer perches above tightly cemented sandstone arrd siltstone o(tlre Raton-Vermejo-Trinidad aquifer. Similarly, many springs are located on the slopes of volcanic capped mesas where water descending through [he volcanic rocks perches at the contact with less permeable sedimentary rocks. Dikes and sills cause springs to develop because the intrusive rocks are barriers to flow, forcing water to the surface. Springs occur in the axial region of lire La Veta Syncline because water in shallow Ilow systems tends to flow down the dip of permeable layers.