Habitat Information

Most people who hear the term “trout stream” automatically think of clear cold unpolluted mountain creeks. This is accurate, but these ideal streams are in danger. Trout also inhabit a variety of water in less than perfect condition due to human interference. After all, the presence of trout is, and has been for many years, used as a measure of water and habitat quality.

A great deal of effort has been expended at rearing and keeping trout healthy. However, few other fish have suffered as much from human activity. Even the most adaptable trout cannot survive a great deal of environmental interference. Trout require cool, clean water and it seems that it is very easy for human activity to interfere with this situation. Logging, housing, or even commercial purposes convert cool, free flowing streams into slow moving, warm, silty waterways incapable of supporting trout. Agricultural, industrial, and domestic sources pollute the water, thereby reducing the oxygen content that trout need to survive. Great strides have been made to reduce obvious sources of pollution, but there is more work to be done.

For example, teaching anglers and non-anglers alike about the importance of habitat. Most people will go out of their way to practice catch and release. We have to assume they practice catch and release to ensure the quality and quantity of the fish. Yet it is these same people that throw their cans and other garbage on the banks. They don’t seem to understand that habitat is ten times more important than practicing catch and release. (Not to say that catch and release isn’t important.) In fact if it wasn’t for the habitat of the stream that trout wouldn’t have been there to be caught in the first place. The following factors play a vital role in the ability of trout and all fish for that matter to survive:


Plankton, the fundamental link in the aquatic food chain depends upon the stream’s fertility or level of dissolved minerals. The source of the water will determine the level of dissolved minerals. Limestone streams have considerably higher mineral content than freestone streams.

A limestone stream is fed by underground springs rich in calcium carbonate and flows over a streambed that supplies even more minerals. Limestone streams have more aquatic vegetation that helps produce more insects and crustaceans, which in turn produces more and bigger fish.

A freestone stream is fed by runoff or springs with low mineral content. The streambed contributes few if any nutrients to the water. Fertile tributaries can contribute nutrients that in turn increase the size and quantity of the fish.


A reliable source of cold water is essential to permanent trout populations. Snowmelt, springs, or glaciers produce this clear cold water. Streams that are fed by surface runoff become very warm during the summer. This doesn’t mean that trout cannot survive under these conditions. Some fish live in streams where the surface temperature in well into the 80’s. However, this causes slow growth rates, reduces active feeding, and decreases their resistance to disease. Other factors play a role in water temperature such as the gradient, amount of shade, and the shape of the channel.


In layman terms gradient means slope. How much the streambed drops for every stream mile. The higher the gradient the steeper the slope, that translates into faster current. The most productive streams have a relatively low gradient. Gradients from 0.5% to 2.0% provide optimal conditions. That translates to a drop of 25 to 100 feet per stream mile.

Stream channels with less than a 0.5% gradient have water that is too warm for trout and streambeds that are covered in silt. Mountain streams with a gradient higher than 7% must have boulders, log jams, pools, or other slack water areas in order to support trout.


Steambeds consisting of clean gravel or rubble produce a greater abundance of insects than a sandy or silty bottom. Sandy or silty streambeds make very poor spawning areas. Siltation is a major problem facing thousands of streams. Silt fills up the spaces between the gravel, which destroys insect habitat and causes eggs deposited in the gravel to suffocate. An overabundance of silt is the result of poor farming practices, overgrazing of stream banks, and logging. Wildlife agencies along with conservation groups such as Trout Unlimited and Federation of Fly Fishers fence streams to keep out livestock and allow vegetation to develop.


Streams with uniform habitat such as a straight and shallow channel produce far less fish than a stream with diverse habitat. Straight and shallow channels prohibit aquatic vegetation which host scuds, larvae and other food sources. Different insects thrive in different stream environments such as riffles and runs. While baitfish and burrowing aquatic insects thrive in pools.

Diverse habitat such as turns, twists, undercut banks, shade, and spawning/resting areas are necessary to sustain an abundant healthy trout population. These streams have great diversity that provides natural cover. This type of habitat provides the classic riffle-run-pool that is critical for trout.


The shape of the channel also plays an important role in the streams ability to sustain permanent trout populations. Deep, narrow channels are better than wide, shallow channels. The wider shallow channel exposes more water to the sun and air causing it to warm quickly. Also as the channel widens it looses its ability to keep silt in suspension. The silt settles and covers gravel beds that provide food and spawning habitat. Stream improvement projects are designed to narrow the channel, increase habitat diversity, repair eroded banks and remove beaver dams.


Most streams can support trout during the spring when water flows are high and the temperature is cool. However, this doesn’t mean that the stream can sustain a permanent population. After runoff the water level drops and the temperature increases. Low water causes several problems; in the summer the temperature rises to extremes and in the winter the stream may freeze throughout the entire water column.

Large underground springs provide the most stability and insure a minimum flow so the stream is unaffected, regardless of the season. Springs also provide a constant water temperature that is cool in the summer and comparably warm in the winter.

In hot weather with little rain the stream loses a great deal to evaporation. Evaporation reduces the depth and slows the current so the remaining water increases in temperature even faster.


Shade from trees and/or overhanging grass helps keep the water cool. Streams without sufficient shade warm rapidly as they move downstream. This limits the area in which trout can survive. Streams with a great deal of shade will hold trout over the majority of its length. However, the cold water temperature inhibits food production, which slows trout growth. Therefore, planting or removing trees to regulate the amount of shade can in part control trout production.


Brown and Rainbow trout can tolerate low water clarity much better than other trout species. Clear water enables trout to see food and also avoid predators. Clear water allows sunlight to penetrate to the streambed, promoting the growth of plants that in turn produce trout food.


Most trout streams have more than an adequate level of dissolved oxygen. Streams with stagnant water or high organic pollutants may not however. Oxygen is replenished through contact with the air.

The ideal stream will have high levels of dissolved oxygen, good water clarity, sufficient shade, a reliable water source, diverse habitat, a snake-like deep channel, a gradient between 0.5% and 2%, fertile water, a gravel bottom and cool temperatures. These factors work in harmony to determine the quantity and quality of fish.

National and international attention is being focused on the problems of toxic contamination and acid rain. With continued public concern and government vigilance, it should be possible for people to fulfill their own needs while maintaining a high quality environment that benefits trout and humans alike. For more information about pending threats and possible solutions to trout habitat visit Trout Unlimited.

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