environmental Controls on Fish Spawning Habitat in Spearfish Creek, Black Hills, SD




Spearfish Creek, in the northern Black Hills of South Dakota, supports the only documented naturally reproducing rainbow trout population in the region.  Until recently, it was unknown where and how much of the creek was available for, and used as, spawning habitat.  This project identifies the effects of natural stream processes on spawning habits and habitats of naturally spawning rainbow and brown trout populations in Spearfish Creek between Hydro No. 2 and Maurice intake dam and in the Squaw Creek tributary.  Spearfish Creek is the main channel of the Spearfish Creek drainage basin in the northern Black Hills.  Originating from springs in the Mississippian-age Madison Limestone, it is super-saturated in calcium and magnesium.  Local environmental conditions interact with water chemistry to produce a variably thick (<1 to 20mm) calcium precipitate rind on the bottom sediment.  The occurrence of the cement rind has rendered a majority of the channel useless for spawning habitat.  However, observed spawning activities indicate limited habitat has existed in zones of cement-free sediment. 

Using detailed geologic reconnaissance, temperature maps and profiles for the creek, geochemical data, and ground-water flow models, areas of upwelling ground water have been identified.  At a nearly constant temperature, these waters (originating from non-limestone formations) have altered surface-water temperature and chemistry at the point of entry into the stream channel.  Thus, upwelling cools and warms the surface water in summer and winter, respectively, and decreases calcium saturation locally, both of which affect calcite solubility and thickness of the calcitic rind.  This process results in limited areas of cement-free sediment correlating to zones of ground-water upwelling, primarily in mid-channel gravel bars immediately above the Maurice intake.  A secondary area of cement-free sediment has been observed on a small gravel delta that has formed at the confluence of Squaw Creek.  Local calcium dilution and warmer water temperatures are maintained by this inflow.  Enhanced by a locally steeper gradient and subsequent percolation of water through the gravel, high spawning activity has been observed on the delta.  In the main channel as a whole, calcite precipitation is greatest during summer and least during winter and is controlled by mixing of ground and surface waters through dilution and temperature stabilization. 

The amount, quality, and location of available fish spawning habitat is a function of environmental conditions.  The chemistry and temperature of upwelling waters continually maintains cement-free zones immediately above the Maurice intake and at the confluence of Squaw Creek.  In addition, high spring flows, which occur after low winter temperatures have damaged (or partially dissolved) the rind have aided in the break-up of the cement, providing rainbow trout access to additional spawning habitat within the main channel.  Conversely, this limits the availability of quality habitat for brown trout, which spawn in fall.  It does not appear that either rainbow trout or brown trout utilizes Squaw Creek for spawning activities.




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