London scoping - ukcip

Final Report
26
Figure 3.13 Number of Thames Barrier closures against tidal surges, 1983-2001
Number of closures per year
14
12
10
8
6
4
2
0
1983
1985
1987
1989
1991
Note: data for 2002 is up to and including 27 April 2002
3.9 Surface Water Quality
1993
The River Thames is now regarded as one of the cleanest metropolitan rivers in the world (EA,
2001b). However, the chemical and biological quality of the London’s rivers and estuary is
variable due to factors such as changing river flows and urban runoff. The General Quality
Assessment (GQA) scheme is used to monitor and assess water quality trends over time and to
compare rivers in different areas. The chemical markers of the GQA are defined by standards
of dissolved oxygen, biochemical oxygen demand and total ammonia. Overall chemical river
quality showed a marked improvement between 1990 and 1995 as flows recovered from the
1989-91 drought, but between 1995 and 1997 quality deteriorated once again as a consequence
of the low flows of the 1995 drought (see Figure 3.14). Summary GQA chemical quality data
for Thames region shows further improvements in quality for the region’s rivers with
approximately 60% of monitoring sites showing improvements in water quality from 1990 to
2001 (www.environment-agency.gov.uk) In line with national trends, river water temperatures
in the Thames have risen since the 1970’s. Rising river water temperatures, particularly in low
elevation and slow moving lengths (Webb, 1996) have also been linked to apparent declines in
some fish populations and changes in macrophyte communities. For example, at the end of July
2000 extensive algal blooms were recorded between Tower Bridge and Putney.
1995
1997
1999
2001