CONFIRMATION OF A CONCEPTUAL MODEL, INTERACTION BETWEEN A STREAM AND A GLACIAL OUTWASH AQUIFER, USING ELECTRICAL RESISTIVITY SURVEYS

Little Kickapoo Creek, a third-order, low-gradient stream, meanders through a 300 m wide glacial outwash valley. The valley lies within a Wisconsonian glacial plain. At the surface, up to 2 m of Holocene Cahokia silt and clay alluvium sits on 5 to 7 m of glacial outwash, known as the Henry Formation. Underlying the outwash is low-permeability clay-rich lodgement till of the Wedron Formation. The stream bed generally runs just below the top of the outwash. Pressure, chemical, and thermal data indicate that stream water is moving through the meanders, thus bypassing the stream channel around the meanders. While migrating beneath meanders, the surface water mixes with the ground water, extending the limits of the hyporheic zone beneath the meander. Although the outwash has been encountered in a number of boreholes, we were uncertain of its thickness and extent throughout the valley. Our working hypothesis was that the outwash was continuous beneath the meander. A series of electrical resistivity surveys was conducted within the valley to delineate the extent of the glacial outwash and to assess the viability of the outwash as a hyporheic corridor beneath the meander. The resistivity results show that the outwash is present beneath the meanders at an elevation that corresponds to the streambed, supporting other evidence of a direct connection between the stream and the outwash. The thickness of the main outwash unit is relatively constant until it pinches out at the edges of the valley. Paleochannels were identified beneath the main outwash unit. The paleochannels are 6 to 7 meters below the stream but are still in contact with the outwash. Within the paleochannels distinct cut banks and point bars can be seen. The orientation of the paleochannels is parallel to that of the stream and bisects the meander. The role of the paleochannels in the flow regime of the area and the possible connection between the stream and paleochannels will be investigated in future work.