The Stream Doctor

January 5th, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. From Kskotey: When a major flood event occurs in a trout stream, I have noticed that the rocks seem to have been cleaned of all algae and that there is a noticeable decrease in invertebrate larvae. Does such an event cause a real decline in stream biomass?

A. Major floods do have severe effects on both the algal and invertebrate communities in streams, but in most cases, the loss is short-lived. Except in the most unusual cases, most spates or floods occur on a given reach of stream, not necessarily from the most upper reaches. This means that the unaffected upstream reaches have the resources to rapidly provide colonizers for downstream, denuded reaches via the mechanism we call drift. This includes both algae and immature invertebrates; they are constantly being dislodged and transported downstream. In most typical cases, you can expect algal populations to be back to normal within a few weeks, with the invertebrates taking a bit longer.

Much depends on the availability of refugia for the organisms, and it is surprising how efficient they are at withstanding severe floods. In a desert spring-stream that I've studied, the stream is normally about 1 m wide and about 5 cm deep. When a flash-flood comes, the torrent is 3 meters deep and probably 15-20 m wide - a lot of water! Yet as soon as it quickly recedes, you can find chironomid larvae and damselfly nymphs within days. Obviously, they found someplace to hide - in the stream bed for the chironomids and by following the rising and receding water levels for the more active damseflies, I suspect. Much also depends on when the flood occurs. If it is during the summer when egg-laying adults are available, they can quickly replenish a new cohort.

If you have a question, please feel free to contact me.
~ 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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