It was a pocket world...
Created in DF v0.28.181.39d.
[WORLD_GEN]
[TITLE:POCKET]
[SEED:391541310]
[HISTORY_SEED:3630883740]
[NAME_SEED:1429538290]
[DIM:17:17]
[END_YEAR:1050]
[BEAST_END_YEAR:30:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:52:52]
[RAINFALL:0:100:26:26]
[TEMPERATURE:25:75:26:26]
[DRAINAGE:0:100:26:26]
[VOLCANISM:0:100:26:26]
[SAVAGERY:0:100:26:26]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[GOOD_SQ_COUNTS:0:4:8]
[EVIL_SQ_COUNTS:0:4:8]
[PEAK_NUMBER_MIN:1]
[OCEAN_EDGE_MIN:1]
[VOLCANO_MIN:1]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:1:1]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:1]
[NON_MOUNTAIN_CAVE_MIN:1]
[ALL_CAVES_VISIBLE:0]
[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:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:5]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:36:72:36]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
This is the relevant river
I did another embark, using Embark Anywhere so I could embark there; this one covered every underground river tile except for the one I had taken up before. At the very top the river is at the surface, about 5-10 tiles in (jagged edge, you see) it goes underground as the ground slopes up. The whole of the river was revealed.
The very top of that map.
Thanks for the quick reply, and I hope this helps!
EDIT: Oh, and I'm going to add that on the 1x1 fortress, mist would frequently burst out from the very beginning or the river. Quite possibly that's the only thing which kept the reclaim party sane when two of them were killed in a cave-in...