I think the verb is "clearing." What the waterways are called varies.

He said the air-cushion vessel will be used for many tasks, including breaking up harbour ice, ***clearing outlets along the St. Lawrence River in spring to prevent flooding***, and deploying buoys and other navigation aids in summer.
http://www.aluminum.org/AM/Template.cfm?Section=Home&CONTENT...

I suppose it's also possible that русла might be specifically "flood control channels". But you probably know better than I do, if there's a different Russian term for that.

(Job description)
Operate heavy crawler or large rubber tire tractors equipped with bulldozer blades and other attachments used in moving earth and other materials, and in maintaining roads, a gravel pit, ***clearing waterways or flood control channels***.
http://www.riversideca.gov/human/jobline/bulletins/Heavy Equ...

--------------------------------------------------
Note added at 13 mins (2010-02-13 20:42:28 GMT)
--------------------------------------------------

Phase one has identified six projects which will
result in an immediate improvement in the
appearance and ecology of the river. We also hope to
open up a new area of public access. The project
sites include George Lane, Stonebridge Lane,
Cooper’s Meadow and Manton Recreation Ground.
The work will include *** river bed cleaning***, bank re-
instatement, tree thinning and vegetation planting.
The partnership has applied to the AONB Sustainable
Development Fund to finance part of the first phase
and we expect work to begin over the Winter

--------------------------------------------------
Note added at 13 mins (2010-02-13 20:42:54 GMT)
--------------------------------------------------

http://209.85.135.132/search?q=cache:yOZc-evOgSUJ:www.riverk...

--------------------------------------------------
Note added at 20 mins (2010-02-13 20:49:25 GMT)
--------------------------------------------------

Duration curves for the sediment transport capacity index for the case of cleaning
the stream bed (critical discharge of 300 m3 s"1) are shown for the three periods in
Fig. 2. The frequency of *** bed cleaning ***has been much reduced. This reduction has
impacted the quality of the habitat because the bed accumulates fines and sand without
being flushed for much longer periods
http://209.85.135.132/search?q=cache:op95QbDDaL0J:iahs.info/...

--------------------------------------------------
Note added at 24 mins (2010-02-13 20:53:43 GMT)
--------------------------------------------------

It can also be used for *** river and sea bed cleaning operations***. In marine diamond mining, mid water mining is limited to fine weather days and good water ...
http://www.yet2.com/app/utility/external/indextechpak/41739

24 Jan 2010 ... She welcomes Port Chester's moves to help clean the Byram River. ... storm-water runoff into the river to avoid polluting the waterway. ... View Larger Image. Jo Conboy stands on the Mill Street Bridge and looks at the ...
www.greenwichtime.com/.../SOS-Cleaning-up-the-Byram-River-s...

Simple, concise, commonly used

?http://www.underwatertimes.com/news.php?article_id=173268540...

--------------------------------------------------
Note added at 1 day2 hrs (2010-02-14 23:01:43 GMT)
--------------------------------------------------

Or "sludge scrubbing out of the river floor", the term automatically means "the channel widening".