Hint 1: If your building a track with a lake or river ...
this is the first thing you do in btb. the river is an open ended track, a lake is a closed loop track with the inside flatten (this will become the Lake later) ...
Hint 2: have a water texture/water material created before you start doing your "project" and export for the first time.
Now when you go to drive on the Lake or water ... you will have to create a section in the "terrain definition file" tbf.
if you want animation ... you need 3dSimed ... and this is done after your track is finished in BTB (there are work-arounds, but requires some outside-the-box creativity)
Hint 3: Don't go crazy in the number of frames used in your animations. Or having too many (large area of animated terrain) close to your driving "racing" path ... this Will suck up valuable FPS ... make sure you have some distance between your river/lake and track.
Hint 4: don't pile on lots of objects in the same area as the animated area for two reasons ... 1] its animated so you want it visable to enjoy it, 2] FPS considerations
ok .... that is enough to get brain cells clicking ...