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| The
Portneuf River |
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| Geography |
Discharge |
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Groundwater |
Water
Quality |
| Groundwater |
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| Groundwater is the water that has percolated through surface soils into underlying layers of rock, gravel, sand, and clay. Groundwater in the Pocatello valley makes up the lower Portneuf aquifer, and it is where we get the water we use to meet our local domestic, industrial, and agricultural needs. Depth to groundwater - how deep you have to go in a well to get a reliable supply of water - varies on seasonal and yearly time scales, in much the same way that discharge in the Portneuf River varies. Just as discharge in the Portneuf increases in winter and spring, due to snowmelt and precipitation, water level in local wells increases as snowmelt and precipitation percolate through soils and decreases during periods when recharge is reduced and as water is pumped from the aquifer. During extended periods of below-average precipitation, such as 2000 – 2003, water level in local wells drops because the amount of water moving into and through the soil decreases. | |
The first figure illustrates seasonal change in depth to groundwater at a well in the lower Portneuf Valley in 1980. Note that the water depth scale is reversed, so depth increases as you go from top to bottom. The level of groundwater decreased rapidly during June of 1980, probably because of large withdrawals from local wells. Groundwater level then increased over the next four months. Precipitation in 1980 (439 mm) was substantially greater than average (290 mm), which may account for the rapid increase in the height of groundwater during the second half of the year. |
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The next two figures show longer term variations in depth to groundwater in two wells that are monitored by the US Geological Survey. In each figure the blue line shows depth to water (note that the water depth scale is reversed, so depth increases as you go from top to bottom), and the red line shows total annual precipitation. |
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Both figures show the annual variation that is more apparent in the first figure. In addition, in both wells the depth to groundwater appears to follow the same trend as annual precipitation, but with a lag of 3 - 5 years. This lag is the result of the fact that water moves through the ground relatively slowly, particularly when compared to water moving down a river channel. |
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| Additional
information about the Portneuf
aquifer is available at: Portneuf Valley Groundwater Guardian Digital Atlas of Idaho - Groundwater Resources ISU GIS Center - Lower Portneuf Valley Groundwater Initiative EPA Region 10 Groundwater Unit Agency for Toxic Substances and Disease Registry (ATSDR) Idaho Geological Survey |
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