Weirs, culverts and barriers

What are the impacts                      WTT advice                WTT do’s and don’ts       

Weir on the Great Ouse - Thomas Smith

Weir on the Great Ouse ©Thomas Smith

Click here for a diagram that shows the habitat impacts of weirs.

Click here for a diagram that shows the geomorphological impacts of weirs.

The impacts of weirs, dams, culverts and other barriers can be divided, broadly, into two main categories: obstructing fish movement and habitat impacts. Both topics are covered in the information below. 

What are the impacts?       

1. Obstructing fish movement

Barriers such as weirs, culverts, hatches and sluices are as great a problem for trout as they are for salmon. It is often assumed that trout do not travel great distances unless they are sea trout, but trout that live in rivers and lakes will migrate upstream to spawn, and drop downstream to find suitable habitat they mature. The video below was filmed by the WTT’s Paul Gaskell and shows brown trout trying to ascend a weir:


As the picture below, shows, obstructions need not be big, just insensitively designed, to pose a problem for migrating fish:


Sea trout attempting to pass culvert, Sussex Ouse

Barriers result in fragmented and degraded habitat, leading to isolated populations of fish. These isolated populations are vulnerable to problems such as pollution or climate change. A fully connected river habitat is much more resilient as it allows natural recovery by migration from trout populations upstream, from side streams or from the sea.

Where practical, removing man made barriers to create a naturally connected river habitat is preferable to installing engineered fish passes which are expensive and require long term maintenance. On many streams, numerous small barriers to migration exist  which can be removed or easements created with the involvement angling clubs, rivers trusts and the WTT . 

A more detailed discussion on the impacts on trout migration of weirs, dams and culverts and their mitigation can be found in the following WTT documents (click titles to access):

The video below is from Trout Unlimited in the USA, but the messages and actions are very relevant for us in the UK.
Weirs and culverts cut off access to habitat for all trout - and including resident  trout that move upstream to access spawning habitat. 


2. Habitat impacts 

Much attention is paid to the "barrier to migration" effect of weirs, but  weirs also have the effect of degrading habitat, often for long stretches both above and below the weir. This is because the transportation of riverbed material is interrupted, causing increased levels of silt deposition above the weir and erosion below. The habitat upstream of a weir is effectively  'drowned'  with deeper, slower water and a silty bed. Often this is not ideal habitat for the flora and fauna that would naturally occupy the river.  Whilst fish passes can be a vital "last resort" that (when they work and are well maintained) may at least allow some wildlife to pass through this degraded habitat; they cannot do anything to improve the habitat itself. Weir removal restores nature's ability to heal itself, and the preferred option, wherever possible, is to remove or modify weirs rather than build fish passes or easements.  

In the video below, the 'Emriver' model and real rivers examples are used to illustrate this aspect of weirs.



WTT advice

The WTT can provide guidance on improving fish passage along with other habitat improvements such as constructing bypass channels. Advice on easements is available in the WTT Information paper Obstructions & Wild Brown Trout, WTT’s Wild Trout Survival Guide & the WTT Upland Rivers Habitat Manual CD all avai​lable on the WTT Publications page (click here to view). Any easement or removal work requires the permission of the relevant authority (NRW, EA, SEPA)  and should be accompanied by appropriate instream habitat management and aftercare as the river bed re-profiles after an obstruction is removed/eased. 

WTT do’s and don’ts

DON’T assume that weirs are providing good holding habitat for fish. A properly functioning river system with sufficient flows will produce a much greater diversity & density of deeper pool areas without the negatives associated with weirs.

DO maintain access for trout, even in small un-fished streams; these are often breeding and nursery areas and can produce the majority of juvenile trout in many river systems.

DO continue to improve instream habitat on your river, in spite of the presence of weirs and their overarching impact on habitat connectivity & quality in a river. It is still important that reach scale problems are addressed whilst long term objectives (free fish passage) are addressed. In this way, when they are resolved, the benefits associated with habitat improvements will be maximised.

DO liaise with the relevant regulatory agency personnel who will establish (and potentially influence) the sequence of fish passage solutions. There are many different designs of fish passes suitable for different locations and species and will require specialist input to ensure their effectiveness. In these cases, angling clubs are usually the catalyst & driver for getting the process underway.

DO involve landowners from the outset and educate them on what you are trying to achieve.

DO ‘encourage’ gradual weir removal. On high energy, spate rivers, weirs can erode away over time. Removing a few choice pieces of the upper structure provides the spate flows something to work with to ‘naturally’ remove the weir.

DO encourage the controlled removal of weirs or create notches in weirs which will allow the upstream river bed to re-align itself gradually.

DO prioritize removal over fish pass provision where possible. Fish passes should be as a last resort as they do not adequately address the problems of habitat connectivity and degradation.