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
Email: streamdoctor@aol.com
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 streamdoctor@aol.com.
Originally published January 5th, 2003 on Fly Anglers Online by Robert Younghanz.