Gaffney Ladce 



Climatic setting. The net annual precipitation balance for Gaffney 

 Lake derived from the MAPS database is -24 inches, toward the 

 middle of the range of evaporative intensity for sites in this 

 class. Based on analyses of evaporation pan data, this is a 

 considerable underestimate Gaffney Lake's water deficit, and points 

 out the need to view the MAPS estimates only as relative indicators 

 of net site precipitation. Mean monthly precipitation for the 

 Medicine Lake climatic station shows a pronounced June maximum. 

 Annual totals for this station show apparent periodicity with a 5 

 to 7 year cycle. Throughout the 1980s, the 3-year running average 

 remained below (often well below) the long-term average, recovering 

 in the early 1990s. In 1993 (the year before sampling) 

 precipitation exceeded the long-term average by more than 1 inch. 



Geologic setting. Gaffney Lake occupies an irregular and modified 

 depression in the seguence of late Pleistocene outwash sand and 

 gravel forming the Clear Lake aquifer, near the contact with kame 

 terrace(?) deposits to the south. The composite thickness of the 

 outwash gravels is greater in the Gaf fney-Medicine Lake vicinity 

 than elsewhere along the Clear Lake outwash channel. Glacial 

 sediments are underlain at depth by the early Tertiary Fort Union 

 Formation. 



Hydrologic type. Gaffney Lake is a natural depression receiving 

 ground-water inflow from the Clear Lake aquifer. A system of 

 ditches and control structures provides managed surface-water 

 inflow from lower Lake Creek by way of "Lake 10" (U.S. Fish and 

 Wildlife Service usage) and other ponds. Managed surface-water 

 outflow occurs via Long Lake to Medicine Lake. Under unusually 

 high stages of Medicine Lake, these flows may reverse, allowing 

 inflow from Medicine Lake (with imported Big Muddy Creek water) to 

 Gaffney Lake. Ground-water outflow to the Clear Lake aquifer and 

 to Medicine Lake is inferred to occur under the typical stage 

 conditions. 



Basin characteristics. The natural surface catchment of Gaffney 

 Lake is small relative to the area of the lake; without water 

 imports, most of the inflow to the lake would be provided by ground 

 water discharge. At management (preferred) stage, Gaffney Lake has 

 an estimated volume of about 4700 acre-feet and an average depth 

 of a little over 7 feet. In 1993 (the year of sampling) the lake 

 elevation was more than 5 feet below management levels all year, 

 with an estimated volume only about 25% of the management-level 

 volume. From 1989 to 1992, Gaffney Lake's estimated water volume 

 never exceeded 55% of the management-stage volume. 



Water chemistry. At the time of sampling, water in Gaffney Lake 

 was highly alkaline, "mesosaline" , and sodium-sulf ate/bicarbonate 

 in composition. Equilibrium calculations reflect estimated 

 parameter values and have no validity for this sample. The 

 ammonium, orthophosphorous and total organic carbon concentrations 



