HYDROGEOLOGY OF KARST SPRINGS IN THE LOGAN CANYON AREA, BEAR RIVER RANGE, CACHE NATIONAL FOREST, NORTHERN UTAH

The Bear River Range in northern Utah consists in large part of a thick sequence (5,000 feet) of Paleozoic-age carbonate rocks that have been locally karstified. These limestones and dolomites contain the principal aquifer in the Logan Canyon area and make up the upper part of the Logan Peak syncline, a large structure that influences ground-water movement in much of the region. All of the carbonate units are capable of transmitting water along dissolution-enhanced fractures, faults, and bedding planes.

Recharge to the carbonate aquifer takes place through discrete points (sinkholes), as stream seepage through fluvioglacial deposits, and as infiltration through soils on ridges and valley slopes. Water entering sinkholes and pits moves vertically downward along solution-enlarged fractures to conduits that channel water to large springs. Precipitation that infiltrates directly probably moves along diffuse pathways through the rock matrix toward conduits and can be a substantial component of long-term storage in the aquifer and base flow of springs.

Discharge from the carbonate aquifer is primarily from large springs along the Logan River, with flow variability from less than 1 to at least 75 cubic feet per second. Spring discharge responds primarily to snowmelt runoff, with peak flow from late spring to early summer diminishing to base flow during the winter months.

On the basis of dye tracing and discharge, recharge areas for the major springs are estimated to be between 5 and 15 square miles and as much as 3,300 feet higher than the springs. Recharge area delineations also indicate that surface-water drainage basins do not coincide with ground-water basins. Maximum ground-water travel times to the springs were 8 to 31 days from losing streams as far as 7 miles away.

Temperature and specific conductance of water from the major springs are inversely related to discharge. During peak flow, specific conductance of water typically ranges from 250 to 290 microsiemens per centimeter (µS/cm) at 25 degrees Celsius (°C) and temperature ranges from 5.5 to 6.0 °C. Conversely, during periods of base flow, specific conductance ranges from 340 to 420 µS/cm and temperature ranges from 6.5 to 8.0 °C. The range of measured values results from mixing of recharge derived from snowmelt with ground water that has had a longer residence time.