A Tale of Two Karsts, Bear River Range, Cache National Forest, Utah

The Bear River Range of northern Utah, composed primarily of a thick sequence (several km) of Paleozoic carbonates with lesser amounts of sandstone and shale, contains at least two alpine karst aquifer systems. Higher elevations in the range are marked by significant numbers of sinks, and members of local National Speleological Society grottoes report a multitude of caves. Spangler (2001, 2004) has shown that the karst is the recharge area for several of the numerous springs issuing along the north side of the Logan River. We are investigating the responses of two of those springs: DeWitt Spring, which is the main water source for Logan, Utah, and which flows into the Logan River from the north, and the spring at Spring Hollow, which enters the Logan River from the south. Chemical and flow data suggest that aquifer characteristics are different north and south of the Logan River. Spangler’s (2001, 2004) data show that the northern part of the aquifer is a very well-developed karst drainage system, with estimated flow velocities on the order of a thousand feet per day or greater. Mineral saturation indices (log(IAP/Ksp)) are indicators of the degree to which a water is saturated with respect to particular minerals. A saturation index of 0 indicates saturation or equilibrium, a negative number indicates undersaturation, and a positive number indicates supersaturation. The mean calcite saturation index in water from DeWitt Spring is 0.2 and for quartz 0.1, slightly supersaturated, consistent with a relatively short period of water-rock interaction. However, the mean calcite saturation index in water from the spring at Spring Hollow, south of the Logan River, is 1.2 and for quartz 2.0, an order of magnitude greater. Such a difference suggests a much lower flow velocity, more time for water-rock interaction, and thus implies a less-well-developed karst system