| Summary: | Anthropogenic water management has extensively altered the worlds’ river systems through impoundments and channel diversions to meet mankind’s increasing demand for water, energy, and transportation. One of these altered river systems is the, now dammed, glacial river Jökulsá á Dal in Eastern Iceland. Construction of the dam (Kárahnjúkar) in the highlands East of Vatnajökull ice cap was from 2004 to 2007. An initial baseline study of Jökulsá á Dal, prior to dam construction, was conducted from 1998 to 2003 in order to constrain the natural discharge regime and fluxes of suspended- and dissolved material. This river monitoring resumed in 2007 in order to assess the impact of damming Jökulsá á Dal and the construction of Hálslón reservoir and Kárahnjúkar power plant on these riverine fluxes. Monitoring lasted until 2013. The dataset collected before dam construction was used to demonstrate natural changes within the river catchments and to assess the effect of climate on chemical weathering rates within the catchment. There is a positive correlation between riverine discharge and suspended load but the correlation is negative between discharge and the concentrations of most dissolved elements (e.g. SiO2, Na, Ca, Mg, DIC, SO4, Cl, F, Sr, Mo). However, some elements are controlled by other factors, such as sunlight (e.g. NO3) and the redox potential of the water (e.g. Fe and Mn). The results showed a positive correlation between the climate parameters (water temperature and river discharge) and riverine fluxes of suspended- and dissolved material, and thus on the flux of many essential nutrients to coastal waters. This potentially adds to the negative feedback between chemical weathering of silicates and atmospheric CO2 through photosynthesis in the oceans. Before damming, the glacial river Jökulsá á Dal carried a large suspended sediment load (~5.8 Mt/yr), the majority of which is now deposited in Hálslón reservoir (~5 Mt/yr). However, the finest grains (<60 µm) remain suspended and are transported with the ...
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