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Why Watersheds are Important

Red ball icon Why Watersheds are Important:  A Lesson from the Rock Valley Project

by J. Michael Gannon and Elizabeth A. Shinall


Alluvial aquifers supply drinking water to approximately 23% of Iowans. In several regions of the state, alluvial systems are the only source of water suitable for drinking. These aquifers consist primarily of shallow, porous sand and gravel deposits along the floodplains and terraces of major stream valleys. They range in thickness from about 10 to over 100 feet.

In recent years, there has been a growing awareness of the vulnerability of alluvial aquifers to surficial contaminant sources. To help communities better protect their groundwater resources, the Iowa Department of Natural Resources has implemented a Source Water Protection Program. Part of this effort involves mapping the area of an aquifer that supplies water to a public well over a specific period of time. Studies near the northwestern Iowa town of Rock Valley in Sioux County will help to refine this process for communities that depend on alluvial aquifers.

In many alluvial settings, there are tributaries entering the floodplain. These tributaries can supply significant water recharge to the aquifer and thus affect the groundwater quality within an area designated for protection. To help evaluate the effects of tributary watersheds on alluvial aquifers, two five-foot hand-driven wells were installed in the channel of Rogg Creek, a tributary of the Rock River just southeast of Rock Valley (see map below). Rogg Creek flows within 400 feet of one of Rock Valley's municipal wells and has a watershed of approximately 16 square miles. Land-use in the Rogg Creek watershed is primarily row-crop production along with several livestock facilities. Both are potential sources of contamination and typical of many Iowa watersheds.

Topo map
The town of Rock Valley draws its water supply from wells in the Rock River alluvial aquifer. Studies show that part of the recharge to this aquifer comes from Rogg Creek and its watershed. The water quality implications of this relationship will be helpful as the community works to protect the source of its groundwater supply. (Rock Valley, Iowa 7.5' topographic map, U.S. Geological Survey, 1968)

The wells were driven through the channel sediments of Rogg Creek and into the valley alluvial deposits of the Rock River floodplain. Monthly water-level measurements were collected from the wells and from the creek. In the eight months between July 1999 and April 2000, the site closest to the valley margin had creek levels (outside the well) between 1.0 and 2.2 feet higher than the groundwater levels (inside the well). This shows that surface water from Rogg Creek is recharging the shallow aquifer near the Rock Valley municipal well field.

  These results suggest that land-use practices in a watershed not only have the potential to impact the quality of its surface creeks and streams, but they also have the potential to impact the groundwater quality of an alluvial aquifer down-gradient of the watershed. In future studies the recharge volume from Rogg Creek will be measured, as will the impact of this recharge on the water quality of Rock Valley's alluvial aquifer.


Adapted from Iowa Geology 2000, Iowa Department of Natural Resources