Investigating the hydrogeological controls of an ephemeral stream’s flow regime on an alluvial fan in an ecologically important setting in North West England

Ephemeral streams play a key role in supplying water and sediment which are of high ecological importance for their permanent tributaries. The upper River Ehen (the Lake District, North West England) is the focus of a habitat restoration initiative to conserve populations of the endangered freshwater mussel (Margaritifera margaritifera). A previously diverted ephemeral stream, Ben Gill, was restored to its original course over a small alluvial fan (0.075 km²) connecting to the River Ehen to reactivate sediment supply and improve the habitat for freshwater mussels. Like most temporary streams situated on alluvial fans, the flow regime and sediment dynamics of Ben Gill are strongly influenced by fan sedimentary characteristics and interactions with its shallow groundwater aquifer.

This study combined high spatial resolution, near surface geophysics and outcrop data with hydrological data to characterise the hydrogeological properties of the alluvial fan and further develop a hydrological conceptual model of the fan to understand Ben Gill stream flow regimes and sediment supply to the River Ehen.

The conceptual model showed the alluvial fan aquifer was highly productive at the fan apex and along buried palaeochannels, whilst reduced aquifer productivity occurred towards the fan margins. When the volume of water entering the fan apex (via a perennial stream) reached ~60l/s, the fan apex infiltration rate was exceeded resulting in ephemeral flows. This typically occurred following rainfall events >9-11 mm. During flows, significant infiltration occurred along much of the ephemeral channel, though a less permeable zone was observed in the mid-fan. In the lower reaches of the ephemeral channel, groundwater levels sometimes exceeded streambed levels resulting in groundwater discharge into the stream during prolonged wet periods. Connectivity between the ephemeral stream and the River Ehen occurs for approximately 20% of the year.

Numerical hydrogeological modelling of the fan is underway to integrate data on groundwater and streamflow dynamics and associated sediment export from the ephemeral stream. This will help gain a predictive understanding of the streams flow regime and its long-term impacts on the River Ehen, which in turn, will determine the success of the restoration initiative.