Streams and rivers in farmland areas often have a degraded morphology due to straightening and run-off pollution (inputs of fine sediment, < 2 mm particle size diameter). Constraints on land use also mean restoration is limited to short sections of watercourse and long term (~10 years) restoration studies are currently limited, making it difficult to assess their success.
In particular, there is a lack of knowledge on the restoration of low energy streams, where the potential for natural processes to form diverse habitat is often perceived to be limited. However, assessing the impacts of restoration could be useful to inform future projects, and this project sought to provide a rare long-term perspective on river restoration.
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A 240m long reach of the lower Logie Burn (catchment area: 25 km2) in Aberdeenshire was restored in 2011 by reconnecting the old course of the channel that was straightened in the mid-20th Century. The purpose was to address several issues in the catchment including diffuse sediment pollution, which is perceived to be compromising the ecological status of Loch Davan within the Muir of Dinnet National Nature Reserve and which the Logie Burn feeds in to.
The specific aims of the restoration project were to:
(1) restore channel morphology, improve river and river bank (riparian) habitat,
(2) reduce fine sediment and nutrient transfer into Loch Davan and
(3) act as a demonstration site for stakeholders to learn about restoration techniques and responses.
In 2011 prior to restoration work, a monitoring program began, and continued until 2020, giving a rare and long-term perspective on the river restoration project.
In particular, the monitoring program sought to determine:
(1) what the styles and magnitude of morphological and bed sediment size change were,
(2) the changes in physical habitat (changes in pool total pool volume and substrate diversity, a measure of the mixture of streambed sand, gravel, organic, wood and aquatic vegetation patches) and
(3) the retention of organic matter, woody material and nutrients (total phosphorous).
In total 8 surveys were conducted over the 9-year monitoring period, before and after restoration, which included several floods that exceeded bankfull level (flooding of the adjacent floodplain; Figure 1).
Figure 1: Mean daily discharge measured at the restoration site. Survey dates indicated by red arrows.
The restored section of the Logie Burn changed shape over the 9 years since restoration with the dominant response being sediment deposition (Figure 2). Bank erosion also occurred in some places as the channel responded to over 20 high flow events that exceeded bankfull (Figure 1). The sediment deposition reflects the reduction of gradient, slowing the flow of water, and the continued high sediment supply from the catchment upstream.
Figure 2: Morphological changes and net sediment volume changes between 2011 and 2020.
Additionally, there was an input of large wood that also trapped sediment. Our observations suggest that the restored reach adjusted to the prevailing sediment and flow regimes quickly due to the sensitive channel state imposed by the restoration work. This demonstrates the importance of anticipating further adjustment in channel form following completion of restoration works. In contrast, the unrestored control reach upstream, showed little morphological change and a smaller gain of sediment, suggesting it was stable and well-adjusted to the prevailing flow and sediment supply regimes.
At the end of the monitoring period, the bed sediment in the restored reach was high in fine (< 2 mm diameter) particles (51% of the sample total) but the overall sediment size distribution was similar to that measured pre-restoration. Restoration resulted in the trapping of fine sediment, with the mean total phosphorous levels increasing significantly from 403 mg kg-1 to 1095 mg kg-1 within the backwaters (stagnant areas of water; see Figure 2 for locations) of the restored reach. Total phosphorous levels however remained similar over time within the restored reach and the control reach (mean of 400 mg kg-1). The retention of total phosphorous within the backwaters suggests these are functioning as sinks that may have a benefit by mitigating the transfer of nutrient rich sediment into Loch Davan thus helping to address one of the aims of the project. By restoring backwaters in other restoration projects we may help to mitigate downstream pollution.
The proportion of the channel comprised of pool habitats oscillated over time but in the end was 100% higher within the restored reach compared to the control reach, and was 20% higher compared to prior to restoration suggesting an improvement (Figure 3A). Substrate diversity, a measure of the variation of bed material (e.g. sand, gravel or aquatic plants), also varied between years but was consistently greater than prior to restoration within the restored reach. In the end diversity was 90% higher than before the reconnection and slightly higher than the control reach in all years (Figure 3B).
Figure 3: (A) Pool volume and (B) substrate diversity (Shannon-Wiener Index) changes over time.
In summary, this study shows that significant and rapid changes driven by floods and large wood input can occur within low energy streams. However, after 9 years of change, it is difficult to assess the overall benefit for ecology with both benefits (e.g., increased habitat diversity and pool habitat) and disbenefits (e.g., deposition of fine sediment) observed.
We are grateful to Dinnet and Kinord Estate and the Muir of Dinnet National Nature Reserve (NatureScot) for site access. The restoration work was coordinated by the Dee Catchment Partnership with the restoration work funded by the SEPA Water Environment Fund. The monitoring and assessment work were funded by the Scottish Government RESAS Strategic Research Programme.
From a restoration practitioner perspective, this case study shows that restored low energy streams are sensitive to change and have sufficient energy to reshape themselves rapidly and create new habitats within timescales of <10 years. This illustrates the importance of expecting and letting ‘the river do the work’ of restoration even in low energy streams.
Whilst our results suggest an improvement in the extent and variety of habitat, this may have been compromised by the deposition of fine sediment to a level similar to pre-restoration. High levels of riverbed fine sediment due to continuing fine sediment supply from upstream, are generally unfavorable for fish and macroinvertebrates. Therefore, our research supports the need for having a catchment wide perspective in restoration projects.
This case study illustrates the types of changes that can occur in low energy, farmland stream restoration projects and this could help with planning in similar settings elsewhere.