GROUNDWATER SPRINGS AND SAPPING VALLEYS IN OTTAWA COUNTY, MICHIGAN

Sapping valleys are valleys eroded by springs and their streams, and contribute to water resources and landform development. Springs along the Pigeon River, in Ottawa County, erode dune sand and form numerous small valleys with the signature amphitheater shape of previously documented sapping valleys. We investigate whether these interesting landforms in Hemlock Crossing Park were formed by spring sapping, and document and quantify sapping processes that include sediment transport and spring discharge.

A detailed map and topographic profile of one valley was created using a total station, and compared to known sapping valleys. Spring discharge was estimated and monitored through the use of staff gauges, and a weir. Two sets of nested wells were installed at the heads of two valleys. HOBO© and Odyssey© data loggers were deployed in wells and streams to monitor hydraulic head, water temperature, and conductivity at ~30 minute intervals. Bedload transport was estimated using sediment traps, and bedload virtual velocimetry determined from GoPro time-lapse images.

Valleys at Hemlock Crossing Park possess most criteria previously described for sapping valleys. Some of these characteristics include: light-bulb shape of basin, dendritic drainage pattern, and high tributary junction angles with the main trunk stream. We observed only small daily and seasonal variations in hydraulic head, temperature, conductivity, and stage in the springs and their streams. Spring-stream flow was ~50 gpm, and stage varied by < 3 cm. Groundwater temperature varied from ~47 to ~56 °F and conductivity varied from ~90 to ~260 µS. Bedload transport estimates are ~0.5 to ~11 m³ each year.

Daily water level variations in wells can be explained by diurnal cycles in transpiration rates of the forest cover. Seasonal variations are hypothesized to depend on seasonal recharge rates of the shallow aquifer and the inherent lag times of the aquifer system. We interpret that these valleys are being formed by spring sapping based upon their morphology, and their observed discharge of water and sediment. These springs and sapping valleys provide cool, moist, and moderating micro-environments for plants and animals along the Pigeon River and are important controls on surface water temperature and chemistry.