How Dams, Geology and Hydrology

Affect Trout Streams


Jimmy D. Moore



     I'm not a fisheries biologist, a geologist nor a hydrologist.  However, I hold a B.S. degree in Forest Management and took several elective courses in Fisheries, Hydrology and Geology while in college. The extra coursework covered fish biology, stream geology and hydrology.  


     What you are about to read is the result of my constant learning process over the years, regarding trout streams and why they act like they do.  I recently finished compiling this bit of work and it has been invaluable to me in my quest to become a better trout fisherman. Being able to read a stream is just as valuable to a trout fisherman as the ability to read a defense is to a quarterback. I enjoy the study of trout streams and I'm happy to share some my findings with you.


     There are many types of trout streams and their geology affects stream hydrology and hydrology affects the species of trout that can survive and do well. Dams also play an important part in stream hydrology and what the stream offers in the way of trout and what species of trout are best suited to it.  Keep in mind that trout are cold water fish and you won't find them where the stream temperature gets much above 75 degrees.  


    Take the meadow streams of the west with their lush green grass, beautiful wildflowers, rainbows and browns, cutthroats and easy wading. Then there are small mountain streams, with their "pocket water" where you rarely wade, but walk from pool to pool hiding behind vegetation along the banks and stealthily flip your fly under an overhang on a pool or run, trying to outwit the cutthroat and the tiny Brook Trout. Perhaps the most beautiful of all trout is the Golden Trout found only in the "ice-cold" mountain streams and lakes of the High Sierras, as we near 10,000 feet in elevation.  


     We all know of the fabulous "Spring Creeks", fed by groundwater emanating from springs flowing out of limestone outcroppings. Spring creeks usually have their "headwaters" in or near the head of a valley floor which leads to a more gentle gradient, (slope), but still have enough movement to oxygenate the water. Spring Creeks are clear and the water quality is superb, which produces lush aquatic vegetation. Rainbow, brook trout, and brown trout abound.


    Freestone streams usually arise from a source other than a spring and are fed by seasonal rains that fill lakes behind dams, whereby the tail waters are cold enough to support cold water fish. Then there's runoff from snow melt, or glacial melt, such as the mountain streams in the Rockies with their browns, rainbows and cutthroat trout.  These streams come rushing down the mountains in a series of pools, runs and riffles as they make their way to the valleys below, where they normally feed water storage reservoirs that ultimately release water used to irrigate crops or for industry and by fly fishers and most importantly, drinking water. There are other streams located in climatic areas where the climate itself prevents air temperatures and ultimately the water temperature from becoming unbearable for the fish. Again, we find rainbows, brook trout and browns.


    The world renowned Chalk streams are restricted by geology to southern and eastern England and are fed by water from Cretaceous chalk aquifers.  Their temperature is mostly constant throughout the year. Chalk streams are rarely subject to floods or droughts.  Runoff is minimal and Chalk Streams are more stable than other types as a result. Chalk streams, like Spring Creeks somewhat limit pool-riffle development because of their gentle gradient. Water quality and clarity is generally good. This promotes a healthy environment for the Brown Trout, insects and aquatic plants.  


     Dams can have a tremendous effect on trout populations.  Not all streams and rivers have dams, but those that do have what is called "Tail Water" or a "Tail Race".  Tail water is that which flows out of the gates of the dam that holds back the stream or river. Dams are built for flood control and hydroelectric power generation and storage reservoirs to hold water for later use. Fishing hydroelectric tail waters is an adventure and can be very dangerous if you're caught on the tail water when water release starts. Hydroelectric dams are required by law to have a warning device that alerts anglers that water release is imminent.  Fishing can turn from great to poor in a matter of minutes because of the rapid rise or fall of the water emanating from the dam.


Rainbow and brown trout inhabit these waters.


     In addition to hydroelectric dams, there are diversion, top draw, (top and bottom draw), whereby the layers of water behind the dam are mixed by alternately releasing from the top and bottom and lastly, bottom draw dams, (dams that release water at the bottom of the dam, whereby the coldest water from the bottom layers of the lake flows out),   In the south, bottom draw dams enable trout to exist where it was once impossible.   Water flowing from a bottom draw dam will be around forty-five to fifty degrees in temperature. This makes it possible for a river to have a thriving trout population in the southern states.  Examples of rivers with bottom draw dams and trout tail waters are the White River in Arkansas, Guadalupe in Texas, San Juan in New Mexico, Lee's Ferry in Arizona, Chattahoochee near Atlanta, and the South Platte in Colorado.


Again, rainbows and browns are common.


     Canyon Lake Dam, on the Guadalupe River near New Braunfels, Texas, the southernmost trout fishery in the United States is a bottom draw and as long as the water flow doesn't fall below 200 cubic feet per second, the water will remain below 70 degrees, even in the long, hot Texas summers.  The result is a self sustaining twelve mile long trout fishery.  I know it's hard to believe you can catch trout in South Central Texas, but you can also catch trout in any tailrace water in the south that has stocked trout and receives cold water from a bottom draw dam flowing at a rate that will keep the water temperature at or below 70 degrees.  


     Some of these rivers, (tail waters), will over a period of years develop a reproducing trout population.  The Guadalupe River Chapter of Trout Unlimited, (GRTU), of which I am a member annually in December and January, stocks the Guadalupe with around 30,000 rainbow trout averaging a pound to a pound and a half.  Later, when we see trout fingerlings in the river, we know they are not hatchery fish, but are from eggs from a female rainbow trout that has made it through the previous year.   The record Guadalupe rainbow is 8 1/2 pounds. While rainbows on the Guadalupe have thrived, we recently stopped stocking browns because they have never reproduced and do not do well.


    Fisheries biologists across the country are experimenting with rainbows raised in lake or stream pens.   These pens hold tiny rainbow trout during the fall and winter months in hopes of teaching them how to feed and better survive the cold winter months. They are fed several times daily and by living in pens actually placed in a lake or stream, they are protected from larger fish and other predators.  In this manner, the fry will thrive and grow and gain knowledge of lake or stream foods that they can eat.


    Pen-raised trout are released into a stream when they reach about a pound in weight.  They are more akin to wild trout in their stamina and are better able to survive simply because their mouths are large enough that they can feed on stream or lake minnows. Planter fish from hatcheries have mouths too small to swallow minnows. Penn-raised trout also have a better flavor than hatchery fish.  Pen raising definitely works, but the problem is having enough large pens to produce enough surviving rainbow fry that grow to release size to make the project worthwhile. GRTU tried this several years ago with a few small pens in the Guadalupe River with limited success.


    If you'd like to know more about fishing Tail Waters, get Ed Engle's book "Fly Fishing the Tail Waters" – Stackpole Books – 1991.  Ed is also a columnist for several US fly fishing magazines.


© Completefisher 2006