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.