The answer is: it mixes the nutrients for plant and animal use and determines a location of organisms.
Current definitely has an effect on biodiversity. But, it does not prevent organisms from living or remaining in rivers and streams, and, thus, cause a decrease in biodiversity. On the contrary, it mixes the nutrients for the organisms and also mixes water less or more saturated with oxygen. Current also determines location of organisms, so, for example, organisms able to resist this current will live in the streams or rivers where current is strong. And vice versa - less resistant organisms will live in the parts of streams and rivers where current is not that strong.