Affects Version/s: Minecraft 13w36a
Fix Version/s: None
Environment:Windows 7 64 bit, Java Version 7 Update 25
The water flowing downwards on the right of the screenshot attached was likely because I snagged water out of the 'lake' with a bucket. When it didn't replenish like it should, I went underwater to investigate and discovered this bug.
I have seen several other lakes in sand mesas and not seen a bug like this.
EDIT: I take that back, there's a long river that runs through the biome and it is the same way.
I did plenty of searching to see if there was a similar bug submitted for this and didn't find anything that looked related.
I checked the remaining water around in the Mesa and all were similar.