Friday, May 20, 2011

Floods and Fly Fishing


High flows are generally not a significant problem for fish in the stream.  Cold water fish like trout have evolved in a system of periods of very high flow associated with snow melt runoff.  Sure some are lost by getting stranded in backwater pools that become isolated as the high water receeds.  But most escape the current associated with high runoff flows the same way they escape them in their everyday existence.  They find shelter in depressions and behind obstructions. If flows haven’t altered the stream bed, when the flow recede they move back into their former preferred locations. (For more information on stream flow and how it effects trout and trout fishing visit these two links http://streamsideadventures.com/seminars/fluid-mechanics-seminar.html   http://streamsideadventures.com/tips-and-techniques/fluid-mechanics.html

The substrate or bottom material of some Missouri trout streams is composed primarily of small, pea to walnut sized, material, sometimes called “chert”, with small scattered areas of larger rock fragments or exposed bed rock.  When streams with this type of bottom material flood, the stream velocity increases to a level high enough to erode the bottom material and move it in suspension or by dragging it along – a process known as traction.   

 It is the movement of the bottom material associated with floods that can cause problems for Missouri’s trout.   When the stream floods and moves the chert it displaces and grinds the fish food organisms – benthos – living on or between the gravel.  When the storm flows recede much of the food base is gone.   A couple of years ago I had an “Ah ha” moment about this on the way home from Missouri’s Current River http://streamsideadventures.com/tips-and-techniques/fluid-mechanics.html.  Even floods later in the year are not going to be devastating to the food base of the stream.  Midges and caddis flies are the primary food source in terms of absolute numbers and biomass.  And most of them have several broods a year.  Some midges can complete their life cycle in no more than a few weeks.  Many of the smaller caddis flies also have short life histories.  So before long there will be food on the table. 

Yes, flooding can slow down the fishing for a little while but it will recover very quickly compared to the devastating effects of drought.  To me the worst thing about a flood event is that the stream has changed so much I have to find new “go to” holes.

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