Sediment study reveals land change impacts on rivers
Sediment regime – the amount, type and timing of sediment inputs and outputs of a river system – can be impacted by land use and land cover changes such as urban development, agriculture and re/deforestation. This can have negative consequences for flooding, hydropower production and even the carbon cycle, since both vegetation and sediment can trap organic carbon by photosynthesis. “Sediment regime has typically been studied by investigating the evolution of bed morphology and sediment characteristics,” explains SEDILAND Marie Skłodowska-Curie fellow Carmelo Juez from the Spanish National Research Council. “The data however tends to be site-specific and limited to short periods of time.” This makes it hard to generalise the precise causes and consequences of long-term sediment regime disturbance in a changing land cover context. To address this, SEDILAND sought to take a more holistic view of sediment changes in river catchments where continuous and increasing land cover alterations are expected.
Sediment regime change
The project, which was undertaken with the support of the Marie Skłodowska-Curie Actions programme, collected and analysed hydrological and sediment records from the past 15 years. This data was sourced from four experimental catchments located in the Spanish Pyrenees. Four study sites were chosen as they share similar geographical and geological characteristics, but with a different gradient of land cover modification. The sites range from relatively pristine forests through to abandoned fields with progressive plant recolonisation and afforested sites. Current and past data – the amount, type and timing of sediment inputs, etc. – were examined to assess long-term trends and cycles. In particular, Juez wanted to better understand how – and how rapidly – land cover changes can impact the sediment regime of a river catchment. Another area of interest was identifying possible time-scale patterns in the production, accumulation and transport of sediment. All this could help to inform better environmental and territorial management and to ensure the preservation of natural processes.
Climate change impacts
The results of Juez’s analysis suggest that land use and land cover changes determine sediment regime disturbance at different timescales. Short timescales are driven by seasonal precipitation patterns specific to the regional climate, while over the long term, alternating periods of increasing and decreasing sediment fluxes maintain the river channel in balance. What is interesting is that moving forward, climate change is likely to have an impact. “Warmer future temperatures, along with more severe droughts and precipitation events, will most likely reduce the relevance of short-term timescales and strengthen the relevance of multi-year timescales,” he says. “Pristine forested catchments with sedimentary discharge dominated by multi-year timescales will thus resist better to climate change. Elsewhere, where short timescales dominate, less sediment will be supplied to river channels, and degradation will likely occur.”
Active mitigation
This underlines the need for forward-thinking land management, in order to mitigate climate change impacts and maintain natural processes wherever possible. These findings should be of interest to ecologists and civil engineers, as well as fishermen and the hydropower industry. Juez would next like to apply machine learning concepts to model sediment fluxes. He hopes this will further encourage his peers to move away from deterministic modelling of river catchments, and towards embracing more random variables in their modelling.
Keywords
SEDILAND, river, sediment, flooding, climate, carbon, hydropower, agriculture, deforestation
