How to get your rivers in the right place

[withlyn]

Lakes are common in some areas, generally those with previous glaciation (Canada, Sweden), tectonic rifting (Africa, Baikal) or an extremely high water table (Florida, Louisiana). Mountain lakes are generally some combination of rift lakes and glacial lakes. Volcanic crater lakes are always fun in an adventure, but are relatively rare within any given region. Elsewhere, most lakes and ponds are formed by manmade dams.

Furthermore, all lakes are subject to eventual siltation. Fast-moving water from the incoming rivers and streams is able to suspend a lot of sediment, but when the water slows down in the lake, that sediment gets dropped, gradually filling up the lake. Depending on the size of the pond/lake relative to the size of the basin it drains (and thus its incoming sediment load), this can take anywhere from a few hundred years (Lake Nasser in Egypt, for instance) to millions of years (Lake Baikal, the African rift lakes). To be truly "realistic" then, a fantasy map should have large numbers of lakes only in regions that are swampy, mountainous, or show signs of geologically recent glaciation.

Although the info about lakes not being found on ridge lines and only draining to one river is generally valid, Isa Lake (actually more of a pond) in Yellowstone National Park is situated on the continental divide at Craig Pass, and actually has two outlets, one which flows to the Pacific Ocean and one which flows to the Atlantic.

Also in Wyoming is Two-Ocean Creek, which splits into two distributaries, one which flows to the Atlantic, and the other to the Pacific.

Just because these things can happen doesn't mean they are normal, or that you would expect them to happen with big lakes or rivers; if you put something like that in a map, make it special!