Capturing Catchment Connectivity Issues

Here at WTT, we're (no pun intended!) all for reconnecting fragmented systems: see recent news items from Tim Jacklin's work on letting the Dove flow, applications of Mike Blackmore's patented #weirbegone, or some of my recent work with Aire Rivers Trust as just a few examples. Europe wide, indeed globally, there is growing recognition of such issues but do we know even the true extent of the problem? Hence, it's great to hear from Siobhán Atkinson regarding her current PhD research.

River connectivity is vital for sustaining healthy freshwater ecosystems. It is important for maintaining resident as well as migratory fish populations, natural sediment movement, and habitat for macroinvertebrate communities and other aquatic organisms. Despite this, few rivers remain uninterrupted across Europe.

Obstacles, or barriers such as culverts, dams, weirs etc. are fragmenting river systems and can be quite extensive. For example, 508 structures with the potential to inhibit fish movement have been recorded in the Nore catchment alone in Ireland. These structures are typically perched culverts, bridge aprons, weirs or fords. To address this issue, an Environmental Protection Agency (EPA) funded project (Reconnect) is being undertaken in Ireland. The goal of Reconnect is to harness knowledge on the distribution, types and impacts of obstacles on Irish rivers, and to develop a methodology for prioritising their modification or removal. The project results will advance efforts to improve the physical and ecological integrity of Ireland’s rivers and will inform the choice of measures to address policy requirements. My PhD focuses on mapping and characterising obstacles in Irish rivers, and studying their ecological impacts.

Figure 1. Examples of different river obstacle types, showing (a) a bridge apron, (b) a natural waterfall, (c) a pipe culvert, (d) a weir and (e) a ford crossing.

The logical first step in addressing the issue of obstacles in rivers is knowing where they are located. This can help managers make informed and targeted decisions on which obstacles to invest removal or remediation measures on. Unfortunately, in Ireland, we do not have a detailed georeferenced map layer of where these structures are. The first part of my PhD is assessing methods of locating obstacles in rivers, and using these methods to map and characterise obstacles in selected sub-catchments. Discovery Series maps, historic maps and satellite imagery are used in combination to locate obstacles. In addition, I am encouraging citizen scientists to use a mobile phone app called River Obstacles ( to help with the mapping.

Figure 2. The selected sub-catchments where I will map and survey river obstacles.

While the impacts of obstacles on fish movement have been quite well studied, less research has been carried out on their hydromorphological impacts, i.e. impacts on the physical environment, and the resultant consequences for fish and macroinvertebrates. Weirs in particular can change the hydromorphology of a river by impounding a length of channel above the weir. In order to understand the impact of this impoundment on the ecology of a river, I am sampling both fish and macroinvertebrates in impounded and in natural river habitats.

Figure 3. One of my study sites; the weir pictured is approximately 3m high, and the sea trout was caught immediately below it.

The final part of my PhD focuses on the use of an alternative, non-invasive and potentially more efficient approach for assessing the passability of a river obstacle, specifically environmental DNA (eDNA) analysis. This involves collecting water samples that contain DNA shed from organisms inhabiting the river.  The DNA is extracted from these water samples and can be used to detect the presence and relative abundance of target species.  I will test the effectiveness of this tool to detect the presence of Atlantic salmon upstream and downstream of specific obstacles. The results of these analyses will be compared with the results of the electrofishing surveys.

Stay tuned for updates on my progress! Please also download the River Obstacles app and join the mapping effort throughout the UK and Ireland.

Siobhán ( or @ShivAtkinson)