Hydrological alterations with river damming in northern Sweden: Implications for weathering and river biogeochemistry

This case study tests the hypothesis that damming leads to a depletion of major elements in river systems. It determines the effect of river dams on the weathering regime, and thus on dissolved silicate (DSi) fluxes from land to the Sea by comparing two headwater areas in northern Sweden. In the pristine River Kalixälven, major dissolved elements are enriched within a few kilometers downstream from a high mountainous provenance, coming from an area low in vegetation and a thin active soil layer to a forested landscape. Also, alkalinity increased from 30 μeq L −1 to 110 μeq L −1 , compared to 240 μeq L −1 measured at the river mouth. In the headwater of the River Luleälven, regulations led to inundation of the river valley and associated loss in vegetated soils. In reaches between the reservoirs, underground channeling of water and a reduction of water level fluctuations result in further decrease in soil‐water contact, and consequently diminishing weathering rates. The ratio of forest area to lentic area in the headwater was reduced dramatically with damming, from 2.65 to 0.84. As a consequence, geochemical variables in the river water show uniformity in space and in time. Alkalinity values at the River Luleälven mouth (155 μeq L −1 ) remained unchanged from the two main mountainous storage reservoirs (161 μeq L −1 and 166 μeq L −1 ). These results indicate that loss of vegetated soils through damming in river headwater critically reduces weathering fluxes and also suggest that changes in vegetation coverage in the Quaternary have altered DSi inputs significantly to the global Ocean.