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[Jhoosier]

I had an underground river that was initially 3/7 or 4/7 deep, and filled with cave crocs, snake/frogmen.  At some point I checked back and all the creatures I hadn't killed/captured were gone, and the water was a full 7/7.  Is this normal?  It's the first UG river I've had, so I've no clue.

[Firnagzen]

The creatures would be gone due to being swept into the chasm, check your creatures page: There should be pages of dead river-stuff there. Not sure about the water levels though.

[Maw]

Do you have the waterfall source tiles for the UG river in your embark?  If so, count the number of waterfall tiles.  Typically, you will have 3,5,5,5,3 tiles = 21 tiles.  This means you have 21 x 7/7 water inflow.  Now, if the river exits on the map edge, you will likely have 5 outlet tiles per z level (I had 10 on my test map - 2 levels).  So, 21 source, 10 exit means the river fills to full very quickly.

[section removed per JG's correction]  Additional info: my map edge exit was 4 zlevels below the source tiles, so everything up TO that level backed up.

Essentially to avoid backup, you need to ensure at least a 21 wide outflow (21 waterfall source tiles remember) - convention states a few more don't go astray either.

Alternatively, if the river comes onto your map, you only need to count the entry tiles (probably 5 tiles per zlevel of the river).  So your source inflow might only be 5 or 10 tiles.  You still need to accomodate that as an outflow.

[Jim Groovester]

Even if the exit is into a UG chasm that is also 21 tiles wide, the river itself is only 5 wide (or 10 if on two z levels).  This restricts the outflow again to 5 (10) tiles max, again causing back up in the UG system.

I have to correct you on this. Water pressure does not work exactly like you describe.

When falling water hits a block of 7/7 water, or when a screw pump pumps into a block of 7/7 water, the block will be immediately teleported to the lowest available empty space not occupied by 7/7 water.

The result of this is that there's no back up in the underground river where the source and chasm tiles are on the same map. The water instantly teleports through the whole system to the chasm, leaving flat sections of 7/7 water that do not overflow.

[Maw]

Even if the exit is into a UG chasm that is also 21 tiles wide, the river itself is only 5 wide (or 10 if on two z levels).  This restricts the outflow again to 5 (10) tiles max, again causing back up in the UG system.

I have to correct you on this. Water pressure does not work exactly like you describe.

When falling water hits a block of 7/7 water, or when a screw pump pumps into a block of 7/7 water, the block will be immediately teleported to the lowest available empty space not occupied by 7/7 water.

The result of this is that there's no back up in the underground river where the source and chasm tiles are on the same map. The water instantly teleports through the whole system to the chasm, leaving flat sections of 7/7 water that do not overflow.

Oops, forgot that