ENVIRONMENTAL CONTROLS ON SPAWNING HABITAT IN SPEARFISH CREEK, BLACK HILLS, SD

Spearfish Creek, located in the northern Black Hills, SD, contained the regions only known population of naturally reproducing rainbow trout. Large yearly fluctuations in their population suggested potential, but unknown, controls existed on available habitat. Spearfish Creek originated from springs emanating from the Mississippian Madison Limestone and was supersaturated in calcium and magnesium. Warm water temperatures during summer promoted precipitation of a calcite rind onto bottom sediments effectively cementing them in place. In winter, as water temperatures decreased, calcium dissolution increased and the rind was weakened. In addition, winter microbial activity formed thin algal layers that alternated with the calcitic rind forming a layered structure; precipitated calcium carbonate alternating with organic-rich carbonate. Two locations did not contain cemented bottom sediments and were where groundwater models indicated upwelling of water from the Deadwood aquifer and at the confluence of Squaw Creek, the single tributary in the study reach. These water sources acted as a dilution to the saturated conditions of the water in Spearfish Creek resulting in local cement free areas which were actively utilized as spawning locations by all trout species. Additional habitat resulted from periodic destruction of the cement rind as functions of decreased water temperature and CO₂ released from algae during decomposition. As a result, the rind was observed to be actively slaking off bottom sediments during early spring, when rainbow trout were spawning, freeing large additional areas of sediment. Increased available and utilized habitat was observed during these conditions. Flow rates were relatively consistent during the study owing to the constant flowing spring source. The only increase in flow occurred during the brief spring snow melt and no observed detriment to spawning occurred due to increased suspended sediment load. Transport of bottom sediment, slope wash factors, alterations to channel morphology, and construction of artificial spawning habitat are the focus of continued research.