in fact, in general the smaller streams tend to have steeper banks than bigger ones, due indeed to rate of flow.
some generalizations:
small streams are most prevalent at high elevations (mountains trigger rains, also melt water)
as water makes it way down to lower elevations, streams merge and become bigger.
at the lower elevations, the slope of the land tends to be increasingly more towards flatland. This slows the flowrate.
Slow flowrate minimizes erosion and causes sediments to settle out. (as vogonpoet mentions this happens in bends, some interesting sickleshaped ponds can be created by this .)
Also (depending on drainage&evaporation) flatlands become swamps/lakes, untill water finds a way lower, then it overflows and erosion will deepen the overflowpoint and the basin will drain.
---
I don't think I like the 7/7 tile digging concept. My own odd/even idea I no longer like either.
I feel the idea of allowing all creatures to climb a single Z cliff would help alot and make ramps superfluous except for use by wagons.
Maybe the size of creatures could have an effect on the traversability of Z-levels, but this would become more complex due to pathing issues.
eg:
marmot size: cannot climb (unless has climbingskill)
dwarf size : can climb 1Z cliff without skillz, skill allows higher climbing.
troll size : can ignore 2z cliffs.
collosus size+ : can ignore 3Z cliffs.
.... .... ....
.... .... ..__
..._ ..__ ..##
__/# __## __##
ramp / 1z-cliff / 2z-cliff