Trout are very sensitive to changes in water temperature. The optimal temperature range for growth and survival of brook trout is 55 to 65° F. Trout streams are often dependent upon their tributaries to supply the cold water that regulate a healthy coldwater fishery. However, many of the streams that feed the Iron River have less than optimal temperatures. Despite the warmer water input from the feeder creeks, a key ingredient to what makes the Iron River watershed so unique is the strong, cold groundwater recharge that flows directly up through the stream bottom. The groundwater recharge that the Iron River receives balances the warm water of the creeks and maintains suitable habitat for brook trout.
Thermal pollution is significant because it affects the amount of oxygen in the stream. The colder the water, the more oxygen the water can hold, and trout need plenty of oxygen. During the summer months, impervious surfaces such as roof shingles, sidewalks, and streets absorb heat as the weather gets warmer. Then when rain events occur, the heat is transferred into the runoff as the water flows over those surfaces. Currently, the watershed is able to cope with the relatively limited number of occasions when this happens. However, as housing and business development in the watershed increases, the impervious surfaces that result will do two things. First, it will increase the area that readily absorbs heat and consequently increase the amount of warm water that will enter the stream during storm events. Second, it will decrease the amount of area able to absorb precipitation and thereby decreases the amount of groundwater available to maintain the cold water recharge.
In the Iron County watersheds some of the creeks such as Nash, Sunset, and Stanley are historically known to have contained viable brook trout populations. Over the last 50 years, changes n the watershed have left these creeks unsuitable for a sustainable brook trout fishery. Disregarding the effects that impervious surfaces and numerous impoundments have in the watershed may lead to a similar result for the Iron River.