Hoghton Bottoms Weir is the largest of many on the River Darwen that create a barrier to fish movement. Structures like this are a problem for fish, like brown trout and salmon, that migrate downstream to find suitable food sources before returning to their spawning grounds in the smaller stretches upstream to mate. Fish use a large amount of energy attempting to get over barriers like this and this affects their success at breeding. Out of all the weirs being tackled through the Ribble Life Together project, alterations to Hoghton Bottoms Weir will re-connect the greatest amount of habitat.
The weir is at the top of a picturesque sandstone gorge. Public access to the weir is good, with a well-used footpath running alongside the left-hand bank of the river. The weir itself is a well-photographed local landmark.
The weir once provided water to Higher Mill at Hoghton Bottoms, as well as Livesey’s Cotton Factory. The mill leat (channel that carried water collected behind the weir to the mill or factory) is now largely dilapidated but is still visible along its entire length from the weir to the viaduct over the river.
Due to its historic importance, large changes to the structure of the weir are not possible. The most feasible option for this is a rock-ramp fish easement (by reducing the steepness of the weir’s gradient, fish will have a better chance of getting up shorter stretches and pools to rest in on the way up). The bedrock outcrop may form part of the channel, over which fish will be able to swim. The main challenge here is the difficulty of access for construction vehicles and materials.
Lower Darwen Weir also poses a barrier to fish movement in the River Darwen. It is adjacent to a swimming pool, pre-school nursery and industrial estate and can be seen from River Darwen Parkway Local Nature Reserve.
Removing the weir is thought to be too high risk for this site due to the structure and condition of the weir and proximity of local infrastructure. Therefore, we plan to construct a rock-ramp bypass channel that will give fish an alternative route around the side of the weir. During high flows, overspill from the river is also beginning to erode a channel behind the weir. The fish pass will help to stabilise the banking at this point and reduce the likelihood of the weir collapsing.
Historically, like many of the weirs on the River Darwen, this weir supplied water to a mill. Maps of Lower Darwen in 1845 indicate that it supplied a mill race for Ewood Cotton Mill (along with several other structures). By 1895, the weir supplied water to the Scotshaw Brook Paper Mill first and then continued to pass to Ewood Mill. This continued until at least 1945 when both mills were demolished, leaving the weir redundant.
Hollins Weir is in a steep wooded valley between Darwen Sewage Treatment Works and the Crown Paint factory on Hollins Road. The weir is a single large beam of hardwood timber spanning two stone channel walls and supported in the middle by several vertical pieces of angle iron.
Fish passage has been improved here by partially removing the weir structure and restoring the section to create a closer-to-nature river channel.
Fish passage has also been addressed at Greenbank Terrace Weir. The majority of the original weir was washed out during a flood in 2012 but a small section of weir remained a barrier to fish passage. When the weir was washed out, it exposed a large area of bedrock upstream and adjacent to the remaining weir. This bedrock was important in limiting the geomorphological impact of the removal of the weir.
Removal of the weir structure has improved fish passage due to the restoration of a close-to-nature river channel, the low risk of geomorphological impacts and the relatively low cost.