Jack McLaren of IDEQ prepares to measure water quality in Island Park Reservoir with a continuous-recording sonde. Photo by Rob Van Kirk.

Plant Material Seems to be Cause of Cloudy Water Below Dam

In recent years, water delivered through the gates on the west side of Island Park Dam becomes cloudy toward the middle of the summer. This year, HFF, Idaho Department of Environmental Quality, and Idaho Fish and Game have been monitoring water quality at three locations below the dam and in the reservoir itself. Preliminary results show that the primary sources of the cloudy water are decaying plant and other organic matter that has been deposited on the reservoir bottom and algae growing in the reservoir. Because decay of organic matter on the reservoir bottom creates a zone of low oxygen concentration, we were concerned about low dissolved oxygen concentrations in the river downstream. However, turbulent flow through the outlet gates appears to oxygenate the water, and dissolved oxygen concentrations downstream have been very high. Delivery of algae and plant material from the reservoir will peak next week during the high-flow event described above and will then subside as reservoir outflow decreases. We will post a detailed report on Rob's blog in a few weeks, once we have laboratory results.