Willem, the cost competitiveness of small scale hydro does not appear to the reasonably attractive compared to large reservoir Hydropower Power and lacks the advantage of Hydropower pumped storage. You could be correct about the cumulative effect of huge numbers on small or micro hydropower installations in most rivers and streams. However, if the environmental impacts are significant, I doubt State and Local Environmental Agencies or affected Residents/Environmentalists would support or allow these projects to be permitted in significant numbers.
My greater concern is preserving the existing Hydropower facilities and net generation capacities. In recent years various Environmental groups have persuaded Governing Agencies to restrict the optimal operation of Hydropower facilities to supposedly restore downstream river systems or curtail adverse impacts on native fish and animal wildlife. This has resulted in U.S. Hydropower net generation stagnating and even directionally declining. Re. EIA MER data.