For size, consider it a cross between the Amazon and Mississippi Rivers of modern Earth. It's headwaters are in the Dawn Sisters at the eastern terminus of the Caspian Valley and it meets the sea at the great Fernwall Bay. This is a straight-line journey of many hundreds of kilometers – and the river tends to curve and loop where the going is flat, like any river will. Portages exist where it's simply more expedient to carry craft overland to avoid a river's loop, saving time and often money.
It is navigable by barge for its entire length. Small towns or trading posts dot the river at regular intervals, each of them denoting one days' travel by barge.
The Caspian River Roads parallel the river on either side (to some extent) and are used for foot, horse, and wagon traffic.
At the head of the river are the Dawn Sisters, a trio of mountains over which spill the various rivers and streams that make up the Caspian. The city of Three Sisters is the hub for all materials shipped out of the region known as Wildermore Range.