I embarked on a ruined goblin tower, to find that one of the obsidian towers had what appeared to be an infinite water source on the roof. It was only two levels tall, built on a hill, and on the wet end the roof was three levels below ground level.
I haven't done any exploratory digging yet, so I'm not exactly sure if it's a cave river, but there should be one in the general area of the tower, but there's a river-shaped wet patch in the wall on the level above the roof. My theory is that the game placed the tower so that it overlapped with the river, and generated some walls to plug the river, which is why I can't see it. Why this hasn't stopped the river from flowing is another mystery.
Running 40d11 with wine, on Ubuntu, with a graphics pack but no other modifications.
I gather that the next version makes serious alterations to how the underground features work, so this might not happen anymore.
Worldgen params, for people who want to replicate this for research purposes or just see what it looks like:
[WORLD_GEN]
[TITLE:WATERPARK]
[SEED:282219198]
[HISTORY_SEED:488966849]
[NAME_SEED:2109306500]
[DIM:257:257]
[END_YEAR:1050]
[BEAST_END_YEAR:200:80]
[REVEAL_ALL_HISTORY:0]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:1200:1200]
[RAINFALL:0:100:800:800]
[TEMPERATURE:25:75:200:800]
[DRAINAGE:0:100:400:400]
[VOLCANISM:0:100:1600:1600]
[SAVAGERY:0:100:1600:1600]
[ELEVATION_FREQUENCY:2:10:1:1:1:10]
[RAIN_FREQUENCY:4:5:3:2:5:10]
[DRAINAGE_FREQUENCY:2:10:1:1:1:10]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:2:25:5:1:5:25]
[VOLCANISM_FREQUENCY:2:20:1:5:10:40]
[GOOD_SQ_COUNTS:100:1000:2000]
[EVIL_SQ_COUNTS:100:1000:2000]
[PEAK_NUMBER_MIN:50]
[OCEAN_EDGE_MIN:2]
[VOLCANO_MIN:200]
[REGION_COUNTS:SWAMP:1032:7:6]
[REGION_COUNTS:DESERT:1032:7:6]
[REGION_COUNTS:FOREST:4128:13:12]
[REGION_COUNTS:MOUNTAINS:8256:9:9]
[REGION_COUNTS:OCEAN:8256:7:6]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:8000:13:12]
[REGION_COUNTS:HILLS:8256:13:12]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:100]
[MOUNTAIN_CAVE_MIN:200]
[NON_MOUNTAIN_CAVE_MIN:400]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:50]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:8256:16512:8256]
[RAIN_RANGES:8256:16512:8256]
[DRAINAGE_RANGES:8256:16512:8256]
[SAVAGERY_RANGES:8256:16512:8256]
[VOLCANISM_RANGES:8256:16512:8256]
Map, for people who don't want to check every goblin site in a large world:
EDIT: I have been informed that these parameters do not generate the correct world. Rather than try to figure out what's up with that, I have uploaded the save file itself, which you can get here.
'NOTHER EDIT: I did some digging, but found no river. The framerate is terrible. It looks like the goblins actually built their tower through an aquifer, which I suppose is also not a good thing. I have no idea why all those underground water critters are there.