Unfortunately there are no rivers in Amnesia, but you can create one with planes as stated earlier.
I think you are looking for the following materials:
water_sewer_streaming.mat
water_sewer_streaming_slow.mat
They are located in the folder
textures\water\
You can adjust the flow speed by changing the material and / or adjusting the tile amount of the plane.
If you want to improve the looks of the "river" you can put something in the way of it and add a splash effect in front of the object.
This will create the effect you can see in this picture:
http://www.moddb.com/mods/possession/ima...1#imagebox
To do this simply add the particle effect
/particles/impact/ps_impact_water_stream_sewer_large.ps (or any other splash effect) in front of the object and you're done.
Feel free to ask if you have any other questions.