River Mimram

The River Mimram in Hertfordshire is one of a number of tributaries of the Upper Lea. Its historic source is to the north of the village of Whitwell. From there it flows south, passing the villages of Codicote, Welwyn, Digswell, Tewin, and Hertingfordbury, before joining the River Lea in Hertford. The river still supports a commercial watercress industry that has been running for almost 200 years, and there are several historic mills dotted along the river’s length.

Problems faced by the River Mimram:

While the Mimram is one of Hertfordshire’s finest chalk streams, supporting fly fishing, populations of water voles, otters, and kingfishers, the river faces many challenges. The flow of the River Mimram is influenced by a number of factors. Consistently dry climatic conditions means that the amount of rainfall which percolates down into the groundwater has declined. The groundwater system provides flow to the Mimram and is also used to meet local drinking supply demands and other abstraction needs.

Another important factor that influences the resilience of the river to extreme weather conditions, like prolonged dry weather or floods, are river modifications. Historically, the river channel of the Mimram was moved to operate a mill. This and other channel modifications have left the river less resilient.

Case Study: Singler’s Marsh

In 2017 Affinity reduced abstraction by a maximum volume of 9.09 million litres of water per day (average 5.6 million litres per day) from the catchment. Affinity Water have agreed to give up their license to abstract water near Singler’s Marsh. While we conclude further investigations into the implications of the full closure of the pumping station near Fulling Mill, Affinity Water have agreed to maintain the operational readiness of the pumping station by abstracting a minimum volume of water. This is to manage the potential groundwater emergence associated with this closure. To offset this small volume, the pumping station near Digswell has been reduced to compensate. Our investigation’s aim is to implement any resulting flood risk measures and fully close Fulling Mill pumping by 2021. Affinity Water plan to reduce the abstraction near Digswell by an additional 6.38 million litres per day December 2024. These reductions are fantastic news for the river’s ecology and we hope to soon see an increase in flow levels, which will mean that the channel will be far less likely to silt up. 

If you’d like to find out even more, follow the link to the Catchment Partnership website for the River Lea Catchment or view the management plan for the River Mimram.

Design by LTD Design Consultants and build by Garganey Consulting.