What other natural communities might be altered because of decreased stream flow?

This question is incomplete. But I can at least list the biological communities that depend on particular stream flow regimes. Aquatic organisms are adapted to particular flow regimes. These regimes tend to be very seasonal in arid and semi-arid regions. So many aquatic organisms have aligned their life stages to co-inside with flow events. Determining flow regimes to sustain aquatic ecosystems is an entire branch of science called environmental flow determination. This science is particularly mature in semi-arid countries such as Australia. This science has evolved because people have built dams on rivers, which has drastically altered downstream flows. Environmental flows can guide how much water should be released downstream of dams to sustain aquatic ecosystems. Various insect species spend their nymph stages in water, such as mayflies and dragonflies for example. So they need to have some flow in a river to survive. Various species of fish also require particular flow thresholds to survive.

The answer is texture