Wyoming Framework Water Plan - Living Rivers Home Page

4.0 RESOURCES
Tensleep Sandstone aquifer of the Bighorn Basin 2 . Groundwater flow is from the basin margins toward
the center and, ultimately, to the north, like the surface drainage.
Figure 4-8 presents typical annual cycles of groundwater levels, in this example from a 110-foot
deep well in an area of active irrigation. It reflects: 1) a pronounced drop in water level each spring, as
irrigation pumping starts; 2) an initially rapid, then slower, recovery of water levels through the fall and
winter as the aquifer is recharged from surrounding areas and the surface; and 3) overall multiyear
fluctuations in the recovered water levels in response to varying irrigation management and climatic
patterns.
In areas away from the influence of irrigation withdrawals, groundwater levels commonly rise in
response to spring precipitation and snowmelt, and fall to annual lows in midwinter due to the absence of
recharge. In aquifers remote from surface influence, there may be little or no annual fluctuation, but
groundwater levels may rise and fall in response to long-term climate cycles. Individual well
hydrographs vary widely, as a function of local hydrogeologic and groundwater development conditions.
4.4.2 Aquifer Classification
Classification of a body of geologic material as an “aquifer” depends on how much water is
needed for a specific user or purpose. A hydrogeologic formation capable of adequately supplying the
modest water needs of a single rural residence may be entirely inadequate to meet the needs of a large
agricultural operation.
Figure 4-9 presents a hydrogeologic map of Wyoming. Approximately 210 individual geologic
formations have been combined into six general groups based on the classifications and descriptions in
the seven Basin Plans 3 . The figure also presents a narrative summary, including reference to the
individual formations 4 within each group.
These classifications are highly generalized, because geologic materials are rarely entirely
consistent over large areas. For example, the Madison Limestone (in the “Major Aquifer - Limestone”
group) is famous for producing flowing wells of phenomenal productivity at certain Wyoming locations,
but in the absence of fractures and solution features will produce very little. Conversely, the Cody Shale
(in the “Major Aquitard” group) is universally understood to be a regional confining unit, but it can
produce modest quantities of water from local sandstone beds. Regional changes in formation
characteristics cause some formations to be classified differently in different basins.
Thickness, yield, and groundwater quality data in Figure 4-9 are bracketed by data from the listed
basins, so may span a wider range than in any one basin. Considerable additional detail on the
characteristics and variations within each aquifer is generally available in the individual basin reports.
The thickness ranges listed are for the formations themselves and do not reflect depths of
occurrence. While most water wells in Wyoming are less than 200 feet deep, wells as deep as 6,000 feet
have encountered useful supplies of good-quality groundwater.
2 Figure 4-7 is from the Wind/Bighorn Basin Plan.
3 The individual plans did not use consistent terminology or formation grouping. The designations used here are compiled/interpreted from the plans’
mapping, tabular, and narrative information.
4 To provide a consistent, statewide formation taxonomy, Figure 4-9 uses formation symbols from the standard, USGS “Geologic Map of Wyoming”
(1985), which is available either in printed form from the Wyoming Geological Survey (www.wsgs.uwyo.edu/) or electronically from
http://www.wygisc.uwyo.edu/24k/bedgeol.html.
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