5.1 SOUTH PLATTE RIVER BASIN
Associated with the South Platte River and its tributaries are silt, sand, and gravel deposits of alluvial and eolian origin that cover an area of over 4,000 square miles. The occurrence and distribution of South Platte River alluvium is shown on Figure 5.1-1. The prominent hogback bordering the west side of the Front Range urban corridor marks the abrupt transition from the geologically complex and rugged terrain of the foothills and mountains of central Colorado to the flatter terrain of the eastern plains. In the mountainous upper South Platte River basin, alluvial deposits tend to be thin and discontinuous and serve as a water resource on a very local basis. Well depths in the upper South Platte River basin, reported in the Colorado Division of Water Resources well permit records, average about 36 feet below ground surface.
In the lower South Platte River basin, east of the hogback and extending across the eastern plains, the alluvial deposits thicken and form a continuous aquifer network that is a major ground-water resource. Grain size tends to decrease whereas sorting and grain rounding increase away from the mountains. Beds of gravel and cobbles, typically found near the base of the alluvium, are less common downstream. The saturated thickness of the alluvium is close to 20 feet near Denver and increases to over 200 feet downstream near Julesburg, the state line (Fig. 5.1-2). Eolian sand and silt cover much of the land surface outside of the stream valleys and overlap the alluvial deposits. Well depths in the lower South Platte River basin alluvium average about 75 feet below ground surface.