Inspired by efreitag’s reference, I looked up some general info here:

http://de.wikipedia.org/wiki/Restwassermenge#Dotierung

and then googled for “dotation + river”, “dotation +water”
This is what I found:

“Minimum” or “reserved” flow must be distinguished by “guaranteed” flow, this last being referred to the release of water (from a specific point as a gate or a valve) in order to comply with specific obligations, regardless of the residual flow in the river downstream of the diversion works. In spite of the lack of simple and unique definitions, the objectives of assuring a minimum flow downstream of hydroelectric installations or other water abstractions is fairly clear.
Anyway we must point out that the problem of a clear definition is important because the definition can greatly affect the value of the reserved flow itself.
We still introduce the auxiliary concept of “dotation” corresponding to the artificially regulated flow rate at a certain time and in a certain cross section to guarantee a required amount of water in a different cross section of the same river."

http://www.esha.be/fileadmin/esha_files/documents/publicatio...

dotation
in an engineering publication from the 19th century: http://tiny.cc/6xhsbw

dotation
in a modern publication (page 64): http://tiny.cc/27hsbw