I designed a fairly simple system for this, using mainly 32x32 baseplates. the entire baseplate is covered by one layer of bricks.
Corners/connections will work like this:
streams will be 8-studs wide, with a one-stud wide plate placed on top of either edge of the river. This only applies to where the river meets the edge of the baseplate it's on. The inner pparts can look hwever you want.
Curved rivers work similarly.
Roads function in the same way(8 studs wide, centered in the edge of the baseplate) as streams, but are put on top of the bricks. they can be paved or dirt.
For dirt roads, tire ruts also work.
Bridges are up to the builder, but the actual road part of the bridge should be 10 studs wide(for medium-sized vehicles)
16x32 plates can also be used, but one side can only connect to similar plates.
Layout so far.
Shores of larger bodies of water. I'm not sure how large the water in question is, nut my shore sustem should work.
Form the edge: Four studs of bricks(to fit the standard corner connection. 2 studs of two stacked plates, and then four studs of beach (once again, this only applies to the edge of the baseplate)
streams can flow in just fine:
Complete layout:
