All streams, at some time, experience very low flow and some may dry up. Some streams except for occasional seasonal pools dry up every year. These are the intermittent streams.
Although they cease to exist as streams during the summer and fall, they are still inhabited by aquatic fauna. However, this fauna does not have an active aquatic life stage during the dry period.
In a very general way, the fauna of intermittent streams can be divided into two groups.
The first is the summer-fall association of the dry stream. The species are few and consist of those organisms that can survive in the remaining pools in spite of the rising water temperature. Or they may be small organisms that can burrow into the moist interstitial spaces below the stream bed where some water still remains.
The second group is the winter-spring association composed of organisms characteristic of flowing water. They appear each year and make up a fairly stable permanent population. By possessing certain features in their life cycle, most members are pre-adapted to escape the unfavorable dry period. For example, the old generation dies when the stream dries up and the population survives as small individuals existing in the moist substrate.
Some invertebrates survive the dry period in the egg stage. Species whose life cycle does not include inactive stages during the dry period are either forced from the area or die as the streams dry up.