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Digitised River Network

During 2000 a new digitised river network (DRN) was developed, based on 1:50,000 ordnance survey data digitised by the Institute of Hydrology. This network ensures consistency between all SEPA areas and includes:

• all mainland and islands rivers with a catchment area of 10 km2 or more. This is known as the 'baseline network'
• mainland and islands stream stretches with a catchment of less than 10 km2, which are classified as fair, poor or seriously polluted and have been monitored. These are added to the baseline network to give a 'classification network'.

It is intended that future emphasis will be placed on the classification network, which is likely to be reportable for the purposes of the EC Water Framework Directive. Efforts to improve the quality of the downgraded smaller streams will continue, but once this has been sustainably achieved, their monitoring may be reduced. Many of these streams are the subject of current attention because of their influence on the quality of larger classification network rivers.

Using the digitised river network scheme, data for every routine sampling point are automatically applied to an identified river stretch of predetermined length. To enable comparison with past changes, it has been necessary to process year 2000 data using both the former system and the new digital scheme. Results for year 2000 using the DRN are shown to the left along with the results from the existing scheme.

Since 2000, SEPA has reported river classification results based on the DRN scheme. The loss in total river length in moving to the DRN (despite the first time inclusion of islands rivers) arises mainly from the exclusion from classification of thousands of small remote headwater streams, which were never previously monitored but which were assumed to be of excellent quality. The smaller reduction in length of downgraded waters arises mainly from using 1:50,000 maps for the DRN; in the former system lengths were hand measured from 1:10,000 maps, so more minor channel bends were included.

In coming years, as well as directing resources to improving poor quality waters, it is intended to extend monitoring coverage using efficient biological methods, to characterise and thus help enable protection of currently good and excellent quality waters. Coastal and estuarine water data will also be digitised.

River classification for 2000 - comparison of existing scheme and digitised river network (DRN)

Comparison of existing scheme and digitised river network