Excellent idea!
The only modification I might suggest is something like:
Water: 13-20 for there to be water.
13-16 Ponds
17-20 Rivers
If Ponds,
1-7 Small Pond
8-14 Medium Pond
15-20 Large Pond
Then, 1-10 Normal, 11-20 add in small Island.
If Rivers,
1-7 One River
8-14 Two Rivers
15-20 Three Rivers.
(Adding in options for ponds/lakes.)