This is a general suggestions as to the behaviour of rivers. Rivers/lakes vere pretty basic in ADOM.

1. Depth.

I think that every square of water should have a certain depth. Depth would be measured in feet, so, for example, trolls could easily wade through a river that could drown a dwarf.

When a character enters the water, the depth would be compared to the character's height, to determine the effect:

Ankle-deep - some speed penalty, but insignificant (1-2 points). Can damage shoes.

Knee-deep - greater speed penalty. Apart from shoes, some types of body armor can be damaged too, like full plate mail.

Waist-deep - now the speed penalty is significant. Most types of body armor can be damaged. You have a chance to trip and fall in the water, exposing your whole inventory to it.

Chest-deep - at this point, the Swimming skill starts to be useful (you can't very well swim if the water is too shallow). Your backpack is submerged. If you trip and fall, you can take some drowning damage.

Neck-deep - if you can't swim, your progress will be very slow. Everything but your helmet is affected by water.

Over your head - if you can't swim, you can still sort of spring from the bottom and breathe, but it's hard - you can traverse the water in this way, but you might take some damage. Everything is affected by water.

Real deep - if you can't swim, you'll most likely drown here, period.

To make this easier, there could be UI mode which would color various depth levels of water with different colors, from green (shallow), to red (deep), so PC could find a good way to wade/swim through a river.

2. Monsters

Monsters would be depth-dependent (i.e. a kraken won't attack you in the shallows, and a giant crab won't pincer you if you can't see the bottom). There should be much more aquatic monsters, probably including tritons and mermaids with rust-proof equipment :) Coral comes in mind as a material.

3. Water walls

Another generic feature might be a part of dungeon that is magically flooded (magically, so the water won't spill to nearby squares). You could enter such place, but you'd take drowning damage, and you would be exposed to water (similar to Water dragon's lair in ADOM). Water monsters could lurk in these "walls".

4. Current
Rivers have currents. There would be some general direction of the current and its strength, defined just once for the floor. Every turn in the river, there is a check of your strength against the force of current and depth (in greater depth, the current acts on bigger part of your body). If you fail the check, you'll be washed away one square in the direction of the current.

4. Liquid types

Of course, once the infrastructure is in place, there can be really ANYTHING in that river, not necessarily water. Various things deal various types of damage and have various effects on your backpack.

Apart from normal water, there can be:

a) Seawater
Slightly more corrosive than normal water, allows generation of sea-type monsters.

b) Very cold water
Deals cold damage. You can encounter seals or penguins here.

c) Hot water
Deals heat damage. You can encounter giant tube worms here, the type that lives in volcanic vents.

d) Trick water
The river is normal water - except that it's tricky. It will always appear shallow, regardless on its real depth. And monsters in it are undetectable until you actually enter the water.

e) Plexiwater
Looks like normal water, but is actually solid. You can walk on the top without a problem. Unfortunately, it's still liquid for the monsters who live in it, who can surface and attack you, but you can't really fight back well.

f) Water of life
A magical river that invigorates everything in it. This rare type won't damage you or your equipment, it will actually heal your wounds. The problem is that the same applies to monsters living in it (mostly good creatures, like angelfish :) ). Even if you kill something in one hit, there is still a chance that the powerful healing magic will actually ressurect it. The magic is in the river, not water, so water taken out from this river will lose its properties.

g) Swamp water
If you take drowning damage in swamp water, there is a chance you'll be poisoned. It's also dark, making it hard to guess its depth.

h) Peat
Pulls you down. If you enter a region of very deep peat, Swimming skill won't save you... you'll need something else. Even if it's not that deep, it slows you down a lot.

i) Quicksand
Likewise. Can pull you down and drown you, slows down a lot. At least it doesn't damage the equipment.

j) Lava
Deals fire damage. Monsters like salamanders and fire elementals live in it.

k) Mud
Slows you down a lot, hard to guess depth.

l) Volcanic mud
Mud with bubbles. Like normal mud, but boiling-hot

m) Anti-lava
Found in weird inverted mountains known as anti-volcanos. Liquefied, freezingly cold rock.

n) Blood
River of blood full of vampiric creatures. Attempts to drain blood of every living creature submerged in it.

o) Liquid darkness
Cold, pure black liquid. What horrors are lurking in its depths?

p) Milk
A river in paradise setting. Satiating, and inhabited by fun-loving creatures. The worst you'll encounter here is a cookie monster.

q) Honey
Seriously, whose idea it was to make a river of honey flow through paradise? Anyone who bathed here must be wary of bears and bees.