How culverts transform rivers in different ways – The FIRE Lab heads back to the Tawe headwaters

In an earlier blog post, I wrote about how the River Tawe harbours a range of flora and fauna despite a long history of neglect and strain inflicted by human activities. While changes in the management of rivers has improved ecological conditions across the catchment, ecosystems supported within the Tawe are still threatened by different human activities (National Resources Wales, 2016).

Culverts, which represent built structures engineered to confine rivers and divert them beneath different human features (e.g. roads, railways), occur at various points within the Tawe catchment. Culverts are known to transform river ecosystems operating across different scales, from changing local habitat conditions (Conesa‐García and García‐Lorenzo, 2013) to restricting the catchment-wide journey that migratory fish need to take from the sea all of the way up to the river headwaters in order to spawn (Januchowski-Hartley et al., 2014).

The FIRE lab is particularly interested in understanding how culverts within the Tawe are influencing river ecosystems. In preparation for future studies, FIRE lab members (myself and Steph) headed to the headwaters to assess different culverts identified from maps that I previously had prepared. We specifically targeted an area of the Tawe very close to its source, where an unnamed road branching off from the A4067 runs alongside the river and over numerous inflowing tributaries (Latitude: 51.863902, Longitude: -3.6692834).

Tawe road culvert study map

The location of the field excursion site: a) within the Tawe catchment and b) along inflowing tributaries – culverts beneath a road (grey) are highlighted by black dots.

Expanding on the interdisciplinary nature of our research, Steph and I were joined in the field by artist Rose Davies, who drew a variety of charcoal sketches of the culverts that we were assessing (see @RosieScribblah on Twitter and https://scribblah.co.uk/ for more details). Much of Rose’s work entails sketching different environments across South Wales. She offered a fresh perspective and outlook on river systems, and was interested to learn about the potential threats culverts can have on freshwater ecosystems.

When scoping out each road-river crossing, what became evident very quickly were the differences in culvert structures on adjacent tributaries, and the effects this had on each river. Certain culvert outlets were perched high above the riverbed, while others more seamlessly connected the Tawe tributaries either side of the overlying road. Such differences are exemplified in the examples below and the amazing artwork drawn by Rose.

Culvert Artist Pic 2 culvert-artist-pic.png

Different culverts constructed in the Tawe headwaters and accompanying artwork drawn by Rose Davies (photo credit @RosieScribblah). One culvert is perched high above the river, while the other matches the downstream riverbed level.

The most interesting culvert effect of the day was saved to the end, where we observed no waters upstream of the road, but a ponded environment downstream of the culvert. Evidently, the structure had severely fragmented the river system, but created a unique ecosystem where fauna such as newts and frogs moved in. These animals love conditions where flows are minimal or absent all together, so this artificially created pond downstream of the culvert provided a unique opportunity for organisms rarely found in rivers to colonize the Tawe tributary.

Frog Newt

The common frog (Rana temporaria – left) and smooth newt (Lissotriton vulgaris – right), which typically inhabit ponds, found in the tributary of the Tawe downstream of a culvert.

The FIRE lab excursion to the Tawe headwaters was insightful and exemplifies the varying effects that culverts can have on river ecosystems, which has significant implications for the conservation of such environments. For example, significant drops from culvert outlets pose a problem for wildlife, disrupting the natural upstream-downstream connectivity of the river and restricting the free movement of animals such as invertebrates and fish (Januchowski-Hartley et al., 2014). Over the coming months, our plan is to undertake a detailed sampling campaign in this region to gauge a more detailed understanding of how culverts affect river environments and the life they support.

We would also like to extend our gratitude to Rose for joining us in the field and producing amazing artwork very quickly! Engaging with different groups and people to highlight the importance of aquatic ecosystems and the threats they face is an integral part of the FIRE lab aims.

Happy Friday all and thank you for reading our blog post!

References

Conesa‐García, C. and García‐Lorenzo, R. (2013). Evaluating the effectiveness of road‐crossing drainage culverts in ephemeral streams. Hydrological Processes, 27(12), pp.1781-1796.

Januchowski‐Hartley, S.R., Diebel, M., Doran, P.J. and McIntyre, P.B. (2014). Predicting road culvert passability for migratory fishes. Diversity and Distributions, 20(12), pp.1414-1424.

National Resources Wales (2016) Tawe to Cadoxton management catchment summary. National Resources Wales Report. Available online through <https://naturalresources.wales/media/679388/2016-updated-_tawe_catchment_summary_nrw.pdf&gt;. [Accessed 7/11/2018].

 

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