WATER QUALITY AND PHYSICAL HABITAT LIMITS IN THE UPPER ANIMAS RIVER, COLORADO

Two factors can limit the population of aquatic animals in a river: water quality and physical habitat. An important source of metals toxicity is dissolved metals in the substrate's pore water and metals attached to very fine sediment in the pore water. The pore fluid and fines within the substrate sediment may have a significant impact on the metals toxicity in the river. In the upper Animas River the toxicity of the metals in the water is the most important limit at this time. Nevertheless, in some reaches of the river there are populations of brook trout. In these reaches the population is limited by a combination of physical habitat and metals toxicity. Winter habitat is also a major limit on the population of fish. Because water and fine sediment in the pore water is an important source of metals toxicity, sediment dynamics is also an aspect of the metals toxicity dynamics. The sediment and metals toxicity interacts with the habitat in a complex and non linear manner to limit the numbers of fish species and size of individual fish.