I think it is capture, explanation of river capture below, they took advantage of this for link up with other forms of transport.

The diversion (capture) of the headwaters of one river into a neighbouring river. River capture occurs when a stream is carrying out rapid headward erosion (backwards erosion at its source). Eventually the stream will cut into the course of a neighbouring river, causing the headwaters of that river to be diverted, or ‘captured’.

The headwaters will then flow down to a lower level (often making a sharp bend, called an elbow of capture) over a steep slope, called a knickpoint. A waterfall will form here. Rejuvenation then occurs, causing rapid downwards erosion. An excellent example is the capture of the River Burn by the River Lyd, Devon, England.

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Note added at 4 hrs (2011-01-25 16:15:42 GMT)
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From the abstract of this paper, looks like it backs up river capture as a valid term here:

Pleistocene carbonate deposits of Central Spain: paleoenviron¬mental relationships
http://oa.upm.es/3366/2/TORRES_PON_1994_01.pdf

Geomorphological and sedimentological
studies, as well as stable isotope analysis and Uranium/Thorium and Amino Acid Racemization dating have made it possible to determine that the Rio Blanco deposits are of Lower (?)-Middle Pleistocene and Upper Pleistocene age, showing predominantly
lacustrine and barrier-cascade-backfilling environmental conditions respectively, this suggesting late capture by the Jalon river.