The Stream Doctor

December 30th, 2002

Your questions and answers about everything stream related.

Q. From John in Colorado...What effects are all of the wildfires in the west likely to have on the ecology of the streams?

A. A good question, and one that would require more space than we have here to completely answer. However, there is a good body of information from the research conducted by Dr. G.W. Minshall and his colleagues from Idaho State Univerity who have extensively studied the impact of the 1988 Yellowstone fires on streams in the Park. I will use his published findings to provide some insights.

Prior to undertaking their studies, the scientists predicted there were be two kinds of effect: (1) short term (1yr) arising from increased water temperatures, altered water chemistry, and change in good quality, and (2) delayed effects, which can be separated into midterm (~1-10yrs) and long term (10 up to 100-300 yrs). Delayed effects would include physical disturbances associated with increased runoff following removal of vegetation by fire, changes in food associated with terrestrial succession, and altered retention capacity of the streams.

Perhaps the best way to describe predicted impacts is to describe what is expected to happen to the important physical, chemical, and biological characterists of streams in both the short term and long term time periods.

Suspended sediments - initial increase followed by gradual reduction as watershed reforestation progresses.

Nutrient output - rapid increase during first year followed by gradual reduction to pre-burn levels after about 10 years.

Incident radiation - increase with destruction of canopy followed by gradual reduction as forest succession proceeds; could result in increased water temperatures depending on size of stream.

Primary production (algal/plant growth) - increase in headwater streams coincident with increased incident radiation and proliferation of algae; gradual reduction as forest succession proceeds.

Litter input - considerable fluctuation as forest develops; related to delayed input of fire-killed snags, new growth, etc.

Shredders (aquatic insects that feed on leaf or other coarse organic matter) - initial reduction as inputs of coarse particulate organic matter from terrestrial landscape are absent; gradual increase in later years as these inputs are reestablished as reforestation occurs.

Grazers (aquatic insects that scrape algae from rock surfaces) - increase after first year coincident with increased incident radiation and algal growth; decrease as canopy developes and shading is reestablished.

Collectors (aquatic insects that pick up fine organic matter from the bottom or filter it from the water column)- gradual increase out to about 5 years as increased inputs of fine particulate organic matter from the disturbed surrounding landscape reach the stream; gradual reduction as watershed becomes more stable.

Each of these predictions considers interrelationshps among several ecological phenomena, and are not just simple cause-and-effect results. For instance, the prediction of increased nutrient outputs, coupled with increasing incident light regimes suggests increased growth of algae (though it will probably change from diatoms to filamentous green algae). However, the rapid growth of understory vegetation in the riparian and terrestrial environments might result in an increased uptake of nutrients by these plants, thus decreasing the predicted increase of nutrient output by the streams.

Well, that's what they predicted, but what have the found? Obviously, results are not completed, but some significant observations have been made. Nitrate-nitrogen concentrations increased in streams of burned catchments. Normal summer water temperatures in small streams were usually less than 15 degrees C, whereas temperatures exceeded 20 degrees C in burned catchements. The stones in burned over streams became more imbedded as the spaces between them filled with sediments from overland erosion. Organic matter in transport and deposited on the bottom of burned-over streams increased.

No major changes were found in the numbers, total weight, and variety of macroinvertebrates, although there were significant changes in the relative numbers of gathering collectors, and grazers at burned sites.

These findings obviously are only the initial things we will learn from these studies. It is hope that the necessary framework has been laid for future scientists to continue to evaluate the impacts of these fires for many years to come.

I'd be glad to furnish more details to anyone wishing to pursue this subject. ~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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