Now, whilst it is obvious that not all virtual rivers will be trying to replicate Earth rivers, it is also likely that whatever world the river is on, life will have evolved to cover the same general niches.

As always, a river can be split into three general areas:

The Upper Reach

The upper reach is characterised as the part of a river that is in the mountains, or very high ground. The fast moving tributaries that gush over the rocks on a fast swirl downhill.

In the upper reaches of a river, algae are prolific. It clings desperately to the rocks, giving them a green, slimy underside, and clawing up wherever water regularly splashes. The water itself is clear of single-celled life - it's just moving too fast for it to survive.

Medium-sized fish (or fish-like creatures) swim here, but only by sheer force of will. On Earth rivers, trout navigate these waters, by flicking their tail back, and curving their body with it, then flicking, propelling themselves against the current like ballistic projectiles.

Birds nest on the banks of the river, eager to feast on insects around the river, or tiny larvae in its fast-moving waters. Such birds (like the Dipper) perch on a stone in the river, watching hawk-eyed. When they see prey, they leap for it, flying, or in the water matters not, and their bodies are streamlined for minimum time in the water.

Such birds do not have the webbed feet ducks and other slow-moving water birds have. In fast moving water, webbed feet are more a hindrance than help.

The Middle Reach

In the middle reach, or river valley, the river broadens out, and as it does, the water tends to flow slower, carving a more gentle, V shaped valley out of the surrounding rock.

As the water slows down, plants are finally able to find root in the riverbanks - the faster moving waters of the upper reach wash away the soil round the roots. Weeds begin to build banks, giving home to wildlife. Strong fish, able to deal with the still quite strong current proliferate. Water snails and softer-bodied larvae begin to make a presence known. Shoals of fish begin to form, and the number of species rapidly expands.

The Lower Reach

Finally in the lower reach we have the delta, or flood plain. The flat lands where the river empties into the sea.

As the flood plain broadens out, the water flows very slowly. Slowly enough for long legged predators to stride across the river, feet on the bottom, and not be bowled over by the current.

Water lilies and other broad-leafed vegetation that reacts so badly to water pouring over the top of it, begin to proliferate. Amphibians such as frogs and toads are able to make use of the current, and birds with webbed feet begin to make an appearance.

The diversity of wildlife is at its height in the flood plain; clams, freshwater shrimps, and other shellfish make a living on the riverbed. The soil is thick and rich, promoting a huge diversity in plant life. Kingfishers or something similar appear here, as do hovering insects. Otters or beavers may live along the riverbanks.

Hot Rivers

Where the river flows through a location that is hot all year round, there are additional types of wildlife to account for. Perhaps the river is in the tropics, or on a world with multiple suns.

The upper and middle reaches are pretty much the same as above, but when you come to the lower reaches, the additional warmth is a catalyst for land animals and birds in the area to use the water to refresh themselves. This results in strong predators making the river their home.

On Earth, some tropical hunters such as torpedo rays, and electric catfish will actually electrocute prey. Electric eels can be eight or nine metres long, and equally fantastic creatures are plausible.

Alligators and crocodiles live along the lower reaches of such rivers - equally powerful predators in other configurations are just as likely. The Anaconda snake lives in the water, and suffocates its prey with its coils, underwater. The matamata turtle hides in muddy waters, and looks like a rock thanks to its shell. Anything edible that comes near suddenly finds the rock has grown jaws. The Anglinga is like a modified heron - it is a bird with a long, thin neck instead of long, thin legs. Sitting on a branch, it can dart down into the water with lightning speed, skewering anything that passes. Piranhas need no introduction.

More docile creatures too, inhabit such rivers. The hippopotamus is a classic example. Anything big enough, in a herd, and that can swim well, or make its huge form nearly invisible, will survive just fine in a river full of predators.