It has been raining steadily for days.  My yard is like a dripping sponge that can’t hold another drop of water.  Nearby streams and rivers are at or over their banks and running the color of chocolate milk.  Flooding and rivers, however, are nothing new.  Basically if you are a river you are going to flood your banks once every few years or more, and in some years experience a flood of major proportions.  The question is not if a flood will occur, but when and how big.

People who live near rivers know all too well the power of water.  If you are an angler and wade frequently in streams or rivers you also know how quickly the force of water increases as its speed and depth increase.  When a river is flooding it can move just about anything in its path and the rocks and cobble on the stream bottom also begin rolling and moving downstream.  If you stand near river while it’s flooding the sound of rolling rocks on the bottom creates an eerie rumble as if the river has some serious indigestion, which in a way it does.

Now imagine you are a small mayfly or stonefly nymph that lives on the rocks along the stream bottom.  A major flood must be like a thousand tornadoes moving through their neighborhood?  How do they survive?  Since floods are a natural part of life in rivers, they must have some solution or these little creatures would have ceased to exist long ago.

First, I should point out that floods do have an impact on the insect life of streams.    Many studies have looked at the density or number of benthic organisms in a stream before and after floods, and they all show fairly steep declines.  In fact immediately after floods the number of insects and other aquatic invertebrates on the stream bottom may be near zero.  But these studies also show that within a month aquatic life is back, though in lessor numbers.  Within two or three months, however, depending on the severity of the flood and type of stream, the numbers of organisms on the stream bottom are often back to pre-flood levels.  Studies also show that some insects are affected more than others.  Chironomids (midges) and other small aquatic Diptera tend to be very resilient and return first.  In general the smaller the organism the better they survive floods and other extreme conditions like drought.  Therefore floods do have an impact and they affect the largest or mature insects the most.  Still, stream life appears to recover quickly, and floods even benefit stream organisms by washing away accumulated silt and debris and redistributing gravels.  No matter if most of the insects that survive are small, the question remains, how do they do it?

A big part of the answer comes from something called the “hyporheic zone.”  The word’s roots are from the Greek words hypo (meaning below), and rheos (meaning flow).  The hyporheic zone therefore means the water that flows beneath the stream bed.  The depth of the hyporheic zone varies widely within and between streams depending on the type of stream substrate - more water flows down and through large loose substrate then fine embedded substrate - and the streams size and gradient.  In some streams the hyporheic zone extends ten or more feet deep and even laterally beyond the edge of the stream banks.  Wherever it occurs the hyporheic region sustains large numbers of aquatic insects and other invertebrates.  In fact studies have shown that the number of insect larvae below the stream bed can equal or exceed the number on top of it.  As a result the hyporheic zone acts as a safe harbor for thousands upon thousands of aquatic organisms protecting them from floods and droughts.  As long as water continues to flow through these subsurface regions of the stream bed the organisms there can continue to survive.  This subsurface flow is also critical to the survival of trout and salmon eggs laid in the streambed, and can even be a safe retreat for small fish fry.

So the next time you see a stream raging down its channel at flood stage, remember that several feet below the stream bed there is a calm region protecting millions of invertebrates and other aquatic life.  Nature has once again provided a solution to what seems to be an impossible situation.