Strictly speaking there are only two types of watercourse i.e. 'ordinary watercourse' and 'main river', but the distinction between the two is not obvious.

'Main river' watercourses are those as designated on a map held and maintained by the Environment Agency.  The remainder are 'ordinary watercourses' and include many drains and ditches

The Environment Agency is responsible for 'main rivers' and the LA (local authority) is responsible for ordinary watercourses.

Prior to about 2005 Critical Ordinary Watercourses (CoWs) i.e. those ordinary watercourses liable to cause flooding were the responsibility of the LAs.

Under existing enmaining legislation CoWs were re-designated as ‘main river’ and responsibility for management of these watercourses then passed from the LAs to the EA in a series of transfers between 2004/6.  The corresponding budget adjustment also transferred funding from the Local Authorities to the EA.

The new Flood Risk Regulations 2009 now identify the LLFA (lead local flood authority) and require assessments, maps and finally flood risk management plans to be deposited - and with regular review thereafter.

I believe that the Thames Region Flood Defence Committee at their meeting on 15 January 2004 considered Agenda item 9 – Report No. T/RFDC/04/03 - a paper on Flood and Coastal Defence Funding Review.

The section on Critical Ordinary Watercourses (page 2 - 4.0.4) states the following:

 These figures (in red) refer to identified CoWs within the Thames Catchment with the specific intention to transfer responsibility from the LAs to the EA.

 I suspect that they are incorrect, and this may go some way to explaining why the CoWs now suffer from lack of maintenance (e.g. the Myrke Ditch)

 Ian Tomes response to my query and dated 8 Sept 2010 may be found here