i enjoyed this dftalk
the retrospective was awsome, downloading 2006 version now
beavers would be possible, but only at high ecological accuracy with a rivers rewrite. Beavers can only dam channels about a metre or two wide. All rivers in DF are about the same width (except at the spring, ive never been to a mouth, are there deltas?). Rivers would have to be rewritten so they change in width to get bigger down stream over distances. Beavers dont eat wood, they eat either plant leaves, or bark, or if they can steal, food from fields.
Most beavers dont actually build dams. They only do this when they are at population capacity within a river basin. Usually they just burrow into the banks and live in a similar fashion to aquatic badgers or large water voles. They only start building dams when no burrowing sites are left. If they cant burrow they build lodges. But they like to exit the lodges into water, so they raise the water level to where ever they can build a lodge. This is usually in very small streams high up the river system. An even here they wont normally build massive dams (although they sometimes do), but just build a dam big enough to create a pool. Beavers are vulnerable to predators out side of water, so the tend to expand their pools if they need to reach wood bark (beavers that live in colder climates are more dependant on bark).