Can artificial floods restore ecological health below the riverbed?

Kate Mathers and colleagues describe their latest research which aims to fill a much needed knowledge gap in the effects of artificial floods on subsurface habitats

Naturally, river systems and their flow regimes are dynamic, reacting to changing meteorological events such as intense rainfall or annual snowmelt. This hydrological variability is vital in supporting a healthy river system because fluctuating flows regulate sediment transport (gravels and finer sediment).

Globally, many rivers are being regulated by dams, weirs and culverts, and it is estimated that only 23% of the world’s rivers are now free-flowing. The construction of dams on rivers often eliminates peak flows leading to a flow regime that is stable throughout the year. These stable and homogenous flow regimes are often associated with impaired ecological health of rivers and, as a result, flow restoration efforts are being widely advocated across the globe.

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An artificial flood was released on the River Spöl over the course of 8 hours with maximum discharge being attained 4 hours into the flood pulse

One such way that is seeing increasing application is the artificial release of water from the dam, which has been implemented in the alpine River Spöl in the Swiss National Park, Switzerland – the focus of our study. Objectives of high-flow releases are system dependent with the aim of the Spöl programme being to stimulate a more natural flow regime to restore habitat properties for the brown trout fisheries.

What did we monitor?

The known ecological benefits of regular artificial floods have historically been evaluated for benthic (riverbed surface) habitats, whilst the implications for habitats below the riverbed (hyporheic) are less understood.

One potential consequence of river regulation is the excessive accumulation of fine sediment (particles <2mm) that limits the ability of many aquatic insects from accessing habitats below the riverbed surface. These subsurface habitats are vital in maintaining the processing of nutrients in addition to supporting a wide range of organisms that can often far exceed numbers in surface sediments.

In our study, we were interested in assessing the ecological effects of an artificial flood release on subsurface habitats.

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Left: looking down from the Punt dal Gall dam, the source of the artificial flood release | Right: installing equipment to sample the aquatic invertebrates living below the riverbed

In September 2018, an artificial flood was released in the River Spöl in the Swiss National Park. The peak discharge of the flood mimicked natural flows that are common during the snowmelt season prior to river regulation. We investigated the effects of the artificial flood at four sites downstream of the dam by sampling various stream properties before and after the flood. For instance, we quantified the amount of fine sediment stored in the surface and subsurface of the riverbed. We also sampled aquatic invertebrates from 0.25 m and 0.50 m below the riverbed to characterise the communities inhabiting subsurface habitats.

What did we observe?

Prior to the artificial flood, minimum flows resulted in excessive fine sediment accumulations within the riverbed that originated from various tributaries entering the river. Following the flood, we found that the fine sediment content of shallow substrates (ca. 0.10 m) was significantly reduced.

The flushing of fine sediment was also apparent in deeper substrates (depths of 0.25 and 0.50 m), which resulted in the reconnection and accessibility of previously clogged habitats. This removal of fine sediment enabled water to penetrate deeper into the riverbed and improved dissolved oxygen conditions.

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The Spöl river had large accumulations of fine sediment prior to the artificial flood

We found that the number of different species recorded within the riverbed was low prior to the artificial flood. The flood increased the number and abundance of species able to access subsurface habitats. The increased accessibility of subsurface habitats post-flood resulted in a change in invertebrate composition 0.25m below the riverbed, although the deeper 0.50 m habitat composition remained similar.

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The artificial flood re-connected the floodplains with the river channel

Our results suggest that artificial floods can be an effective management tool to flush out fine sediment from riverbeds, improving the health and accessibility of subsurface habitats for aquatic invertebrates. We anticipate that artificial floods are needed on a regular basis due to the relatively rapid re-accumulation (10 months post-flood) of fine sediment in the riverbed. This finding is in line with previous research conducted in the Spöl that has documented that regular artificial floods (annually) are required to maintain the ecological benefits of flood pulses for riverbed surface habitats.

Read the full research article: “Artificial flood reduces fine sediment clogging enhancing hyporheic zone physicochemistry and accessibility for macroinvertebrates” in Issue 2:4 of Ecological Solutions and Evidence.

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