Riverfly sampling at recently regravelled sections of the Mimram is showing that invertebrates are already colonising the new habitat!

In November and December last year, a project was undertaken to improve the river bed habitat of the Mimram at Singlers Marsh Nature Reserve, on the outskirts of Welwyn. Owned by Welwyn Hatfield Council, Singlers Marsh comprises a section of regionally important chalk river and lowland flood meadows, and is one of the few sections of the Mimram open to the public. However, in recent years the riverbed quality has dropped, due to low river flows causing silt to accumulate and cover the gravel.

The project, funded by Natural England via the Lea Catchment Nature Improvement Area and managed by HMWT in partnership with Friends of the Mimram, the Environment Agency and Welwyn Hatfield Council, involved removing silt from four five-metre-long sections of the river, and replacing it with gravel from Panshanger quarry. The resulting 'riffles' were intended to provide essential habitat in which invertebrates could live, and fish such as the Brown Trout could spawn.

The latest results from the Riverfly Monitoring Initiative (RMI; previously known as the Anglers Monitoring Initiative) monitoring of this site are encouraging.  The higher the score, the better the site; the total score for the site was 9, but when broken down across habitat the RMI score was 10 for the newly gravelled areas, and 7 for the unrestored silted areas. The big difference in scores was due to increased numbers of Gammarus pulex (freshwater shrimp), cased caddis and caseless caddis in the restored sections, by factors of 3-8. 

Although only four of the eight target species groups were recorded, the results so far are certainly promising, showing that invertebrates have already moved in to colonise the restored areas. Additionally, water starwort has been spotted on the regravelled areas, which will provide additional cover for wildlife.