All the rain gives us the River Mimram back, but where are the fish?
The River Mimram is one of the world’s rare chalk streams. (There are only 220 of them.) And they are fed by underground aquifers which are replenished by steady, winter rains. However, after three very dry winters, our aquifer was almost empty and the river dried up. (Affinity Water had already reduced the amount they abstract from Singlers Marsh and have not increased it since.)
The Environment Agency (EA) measure the groundwater levels of the Mimram at Lilley Bottom and you can see from their chart below that the groundwater was exceptionally low in September. But now, after all the rain this Autumn and Winter, it has now risen but it is still below average. Provided we get an average rainfall fall for the next two months, this should be enough to keep the river running through the summer.
One other aspect of the dry winters was that by last Spring we were in an official drought and Affinity Water were expecting to have to introduce a hosepipe ban (Temporary Use Ban) this April. That will no longer be necessary, but their Drought Committee will continue to monitor the situation.
And what about the fish in the Mimram? Last September, as the river levels dropped dramatically, the EA rescued the fish and rehoused them downstream. Now we have to wait for the riverflies to return, who are the bottom of the food chain. Once they are back then we hope the fish will return and be sustainable. However, due to the large weirs at Tewinbury and Mill Lane, the fish cannot migrate upstream as they would do naturally.
So there are three options. Some fish did survive upstream above Codicote and they might come down stream to Welwyn. The EA could restock the Welwyn stretch with breeding fish. Or the EA could bring some of the fish stock back from Panshanger. The EA plan to monitor the situation closely and make the decision, but don’t expect to see anything much for three to four years.
In the meantime, enjoy the Mimram Mimram at Singlers Marsh. Or, if you fancy a walk, there is a very good stretch at Panshanger Park, near Hertford.