Preliminary results of a study of sapping valleys in Ottawa County, Michigan
Sapping valleys, Michigan, Hemlock Crossing, groundwater, streams, sapping, springs, discharge, bedload transport, water level, hydraulic gradient
Larson, Elanor A.; Riemersma, Peter E.; and Colgan, Patrick M., "Preliminary results of a study of sapping valleys in Ottawa County, Michigan" (2019). Student Summer Scholars Manuscripts. 206.
Sapping valleys are valleys eroded by springs and their streams and contribute to water resources and landscape evolution. Springs along the Pigeon River, in western Ottawa County, erode dune sand and form numerous small valleys with the signature amphitheater shape of previously documented sapping valleys. These springs are important for providing clean and cool water for micro-environments and the larger watershed. We investigate whether small valleys in Hemlock Crossing Park were formed by spring sapping, and document and quantify sapping processes that include spring discharge and sediment transport.
A detailed geomorphic map and topographic profile of one small valley was created using a Trimble M3 total station and data were mapped in ArcMap 10.3. A high resolution digital elevation model, our geomorphic map, and topographic profiles were then compared to previously described sapping valleys, to distinguish common morphologic traits and test whether this small valley is likely formed by sapping processes. Spring discharge was estimated and monitored with staff gauges, and a 90-degree notch weir. Two sets of nested wells were installed at the heads of two small valleys near spring heads. Staff gauges and wells were monitored from May to August of 2018. HOBO© and Odyssey© data loggers were deployed in the nested wells and stream beds to monitor hydraulic head, water temperature, and conductivity at ~30-minute intervals over the ~4 month period. Bedload transport was estimated using sediment traps and bedload virtual velocimetry determined from GoPro time-lapse images.
Three small valleys at Hemlock Crossing Park possess most criteria previously described for sapping valleys. The valleys possess seven out of the eleven characteristics including: light-bulb shape of basin, low basin-area to canyon- area ratio, theater or cirque-like valley heads, steep valley walls and flat valley floors, 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, discharge, conductivity, and stage in the springs and their streams. The hydraulic head and stream discharge increased in early May and then steadily declined from late May to late August. Temperature of the groundwater increased gradually over May to August. One set of nested wells (site #2) indicated a gradual overall decline of upward gradient, while the gradient in another set of nested wells (site #1) declined more rapidly, until it was zero by mid-August. Because of an increase in precipitation at the end of August, the gradient at site #1, indicating recharge of the aquifer. The wells at site #2 show a decline in gradient, but the groundwater still flowed upward at the end of August.
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. Bedload transport rates are notoriously difficult to estimate, but our virtual velocimetry measurements and volumetric sediment transport calculations suggest that valley volumes could be excavated given the rates we calculated. We observed very low bedload transport rates during the month of August. The flow at that time was so low that little bedload was being transported. 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.