Well, I have found a number of freaking AWESOME sites along this river so far. One has a huuuge abundance of magnetite but no flux, one that I just genned has tons of cassiterite, galena, and marble but no natural iron (goblinite would work), plus an abundance of river-dwelling creatures including platypus-men/women and alligators (said map started out thawed). All of these maps have huge vertical cliffs, which seem to be a feature specific to The Great River 'Babyseize'. Babyseize is also chock-effing-full of waterfalls created by smaller rivers cascading into its canyon.
For your convenience, here are the worldgen parameters, so you can explore and find sites of your own. I'm going to keep looking to see if I can grab one that has iron + flux + other metals, and if not I'll probably go back to the one I just explored with the alligators (for breeding purposes, naturally). I've been going with The Hatchet of Reverence as my civ, since it has a pretty good amount of materials available for selection.
I think I'm going to rename this topic to reflect the fact that it's now about a really freaking awesome river.
Worldgen params:
[WORLD_GEN]
[TITLE:LARGE REGION]
[SEED:57622548]
[HISTORY_SEED:184235100]
[NAME_SEED:312046360]
[CREATURE_SEED:2884103632]
[DIM:257:257]
[EMBARK_POINTS:1274]
[END_YEAR:1050]
[BEAST_END_YEAR:300:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:400:800:800]
[RAINFALL:0:100:400:400]
[TEMPERATURE:25:75:400:400]
[DRAINAGE:0:100:400:400]
[VOLCANISM:0:100:400:400]
[SAVAGERY:0:100:400:400]
[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]
[MINERAL_SCARCITY:2500]
[MEGABEAST_CAP:75]
[SEMIMEGABEAST_CAP:150]
[TITAN_NUMBER:33]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:52]
[NIGHT_CREATURE_NUMBER:52]
[GOOD_SQ_COUNTS:100:1000:2000]
[EVIL_SQ_COUNTS:100:1000:2000]
[PEAK_NUMBER_MIN:50]
[PARTIAL_OCEAN_EDGE_MIN:2]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:15]
[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:8256:13:12]
[REGION_COUNTS:HILLS:8256:13:12]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:40]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:1500]
[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]
Also, here's a picture of the general area of Babyseize I've been checking out:
As you can see, Babyseize is characterized by huge cliffs. It's like this for pretty much its entire length in this region, as far as I can tell, so I'm assuming it's the DF equivalent of the Grand Canyon. If you want to look for a cool embark site, the best way to do it is just to look for a point of flat land (aka 1's and 0's) jutting out into those sheer cliffs.