Twin Lakes are located on the eastern slope of the Rocky Mountains in western North America. They lie at an altitude of 2,804 m above mean sea level, in the Montane Life Zone of Colorado. Twin Lakes are a pair of connected lakes which were originally formed by the morainal damming of Lake Creek by a Pleistocene glacier. This creek, a small tributary of the Arkansas River, was the main source of inflow to the lakes in their natural state.

The Lake Creek watershed is relatively small, with an area of only 238 km2. While the lakes themselves lie in deposits of glacial till, Lake Creek drains mainly the crystalline rocks and eroded ore bodies of the eastern slope of the Sawatch Range, along the Continental Divide. Consequently, the deeper sediments of both lakes, which are composed of a fine glacial rock flour, have accumulated large quantities of heavy metals, including iron, manganese, copper, zinc, lead, and cadmium.

The climate here is cool and semi-arid. Twin Lakes are dimictic, with an ice cover usually lasting from early December to early May in an average year. Maximum ice thickness ranges from about 60 cm, in an average year, to a meter or more during severe winters. Summer thermal stratification normally reaches its maximum in August, with surface water temperatures in the range of 15!|to 18!|, and bottom temperatures of about 9!|.

Upper Twin Lakes had a normal surface elevation nearly 2 m higher than the lower lake, and the natural fluctuation of surface elevation in each lake probably did not exceed 0.6 m annually. The normal surface area of the upper lake was about 220 ha, with a corresponding maximum depth of about 25 m. Lower Twin Lakes was larger in surface area, about 610 ha, but more shallow, with a maximum depth of about 23 m. Both lakes were probably oligotrophic, but they harbored diverse zooplankton and fish communities. One species of native trout was described as being endemic to Twin Lakes.

Permanent settlement began in this area in the late 1870's, and almost immediately the settlers began changing the ecology of Twin Lakes through the introduction of exotic species, and by modifications of the hydraulic regime, which were aimed at converting the lakes into water-supply reservoirs for irrigation and mining.

Since the turn of the century, a series of hydraulic engineering works had converted Twin Lakes into a pair of connected reservoirs. First, the natural outlet of the lower lake was dammed, and a deeper, gated outlet was constructed about 1 km to the north. This arrangement allowed a potential vertical fluctuation of 7.8 m in the water surface elevation of the lower lake. Next, the stream connecting the lake was dredged into a channel that allowed the two bodies of water to fluctuate essentially as one. Finally, a tunnel was constructed under the Continental Divide to divert water from the western slope into Lake Creek. This transmountain diversion increased the total annual discharge of Lake Creek by an average of 42%. It is important to note here that this additional inflow was all added during the irrigation season, which extends from approximately June through September. Thus, the extra flow had the effect of increasing and prolonging the natural runoff peak in Lake Creek.

The net result of these hydraulic changes was increased erosion in the inflow area of the upper lake. What had originally been a marshy meadow was now an eroded floodplain, and the resulting woody debris was deposited in the bottom of Upper Twin Lakes. During severe winters, with prolonged ice and snow cover, this allochthonous organic deposit exerted a biochemical oxygen demand that resulted in extreme hypolimnetic anoxia, with toxic metal releases from the sediments. Primary production in the upper lake was also limited by intense flushing and turbidity during the spring runoff period. In fact, the upper lake functioned as a settling basin and buffer for the lower lake (1).