Anthropogenic Reservoirs of Various Sizes Trap Most of the Sediment in the Mediterranean Maghreb Basin

Abstract

Abstract:The purpose of this study is to obtain a spatially explicit assessment of the impact of reservoirs on natural river sediment fluxes to the sea in the Mediterranean Maghreb Basin (MMB), a region where both mechanical erosion rates and the anthropogenic pressure on surface water resources are high. We combined modeling of riverine sediment yields (sediment fluxes divided by the drainage basin area) and water drainage intensities in a 5' × 5' grid point resolution (~10 km × 10 km) with a compilation of existing reservoirs in the area, and calculated sediment trapping based on average water residence time in these reservoirs. A total number of 670 reservoirs could be assembled from various sources (including digitization from Google maps), comprising large-scale, small-scale and hillside reservoirs. 450 of them could be implemented in our modeling approach. Our results confirm that natural sediment yields are clearly above the world average, with the greatest values for Morocco (506 t km −2 year −1 ), followed by Algeria (328 t km −2 year −1 ) and by Tunisia (250 t km −2 year −1 ). Including dams in the downstream routing of suspended sediments to the sea reduces the natural sediment flux in the entire MMB to the sea from 96 to 36 Mt km −1 year −1 , which corresponds to an average sediment retention of 62%. Trapping rates are highest in the Tunisian basin part, with about 72%, followed by the Algerian (63%) and the Moroccan basin parts (55%). Small reservoirs and hillside reservoirs are quantitatively important in the interception of these sediments compared to large reservoirs. If we only considered the dams included in the widely used Global Reservoir and Dam (GRanD) database which comprises mainly large reservoirs sediment retention behind dams would account for 36% of the natural suspended particulate matter (SPM) flux to the Mediterranean Sea. Our data reveal negative correlation between sediment retention and natural erosion rates among the three Maghreb countries, which can be explained by the greater difficulties to build dams in steep terrains where natural sediment yields are high. Although the lowest sediment retention rates are found in the Moroccan part of the MMB, it is probably here where riverine sediment starvation has the greatest impacts on coastline dynamics. Understanding the impact of dams and related water infrastructures on riverine sediment dynamics is key in arid zones such as the MMB, where global warming is predicted to cause important changes in the climatic conditions and the water availability.

Long-Term Water Quality Patterns of a Flow Regulated Tropical Lowland River

“…The nutrient levels increase again as the river flows through the downstream floodplain through input from nutrient rich soils and fertilizers. The drought did not have any significant effect on water quality as the PAI remained similar to pre-drought conditions.Water 2020, 12, 37 2 of 22 impacts on the environment, as well as on the biophysical characteristics of a river both downstream and upstream of the impoundments [7][8][9][10]. Effects include altering the magnitude and velocity of floods and river flows either upstream or downstream of the dam [11], sediment trapping within the impoundment [10,12,13], loss of fertile topsoil downstream of the dam [8,14], nutrient retention within the impoundment [13], nutrient loading downstream of the dam and loss of biodiversity, particularly fish and macroinvertebrates [9,15,16].…”

“…The drought did not have any significant effect on water quality as the PAI remained similar to pre-drought conditions.Water 2020, 12, 37 2 of 22 impacts on the environment, as well as on the biophysical characteristics of a river both downstream and upstream of the impoundments [7][8][9][10]. Effects include altering the magnitude and velocity of floods and river flows either upstream or downstream of the dam [11], sediment trapping within the impoundment [10,12,13], loss of fertile topsoil downstream of the dam [8,14], nutrient retention within the impoundment [13], nutrient loading downstream of the dam and loss of biodiversity, particularly fish and macroinvertebrates [9,15,16]. The type and magnitude of these effects may differ between systems and is highly dependent on the size of the dam, whether hyper-or hypolimnetic discharge occurs, the purpose for which it was built, the type of river it is located on and the natural variability and function of rivers [12,17,18].…”

Paris Climate Agreement: Promoting Interdisciplinary Science and Stakeholders’ Approaches for Multi-Scale Implementation of Continental Carbon Sequestration

“…For instance, few studies quantify C content in sediments and compare input and output of sediments in and from dams' reservoirs. Recent results in Maghreb show that 55 to 72% of natural specific riverine suspended matter (SPM) are prevented to reach the sea since the 80s, [36]. Moreover, in recently setup dam's reservoirs, submersion of organic carbon contained in original soil and forests is followed by significant upstream and downstream emissions of CO2 and methane that may compensate for C fixation in sediments [37].…”

“…We should endeavor to harmonize and standardize data-acquisition methods to improve quality of the data and make them easier to use with existing book-keeping C models [39]. Methodological studies of measurements of C stocks and GHG fluxes are required to improve accuracy and acquisition speed, and to reduce costs [36,41]. Because CoCS encompasses a wide range of ecosystems, soil types, plants, and management practices, we will need many different types of measurements and, sometimes, specific measurement methods (not to mention safe access to fieldwork sites).…”

Modeling and Practice of Erosion and Sediment Transport under Change

“…The Mediterranean Maghreb Basin (MMB), that extends over Morocco, Algeria, and Tunisia, is a region where both mechanical erosion rates and the anthropogenic pressure on surface water resources are high and were subject to analysis [48]. Based on sediment trapping, calculated by the models using information and limited data from 470 out of 670 reservoirs in the area, it is confirmed that natural sediment yields are clearly above the world average, with the largest being in Morocco and the smallest in Tunisia.…”