Both before (1967-69) and after (1976-78) the regulation of the river Ekso in western Norway, physical and chemical analyses were made of the river water. After the regulation water samples for chemical analyses were taken at the inlet and outlet of a weir basin 375 m long which had recently been built to maintain the previous water level.

The reduced water discharges and the increased water temperatures which followed the regulation presumably increased the amount and the quality of food available to detritusfeeding animals.

The O2 content of the water was slightly reduced after the regulation. The pH was in the same range. Specific conductance (H20) and the concentrations of major ions before the regulation (Ca2+, Cl NO3-N, PO43−-P) and after the regulation (Ca2+, Mg2+, Cl, SO42−, N03-N. NH4+-N, PO43−-P) were also in the same range, but a distinct seasonal variation appeared after the regulation.

These variations were thought to have three main reasons: 1) water discharge, 2) biological production, 3) nonspecified physicochemical relationships.

Ionic fluxes through the weir basin were highest during the autumn. Only small differences were found between the inlet and outlet of the weir basin.

Regression analyses based on the concentrations of major ions and water discharge after the regulation were made separatly for the summer and the winter season. H20 and SO42−-concentrations were not correlated to water discharge, Cl-concentrations were positively correlated, whereas Ca2+ showed a negative correlation. Mg2+ and NO3-N were not correlated to water discharge during the summer season, but showed a significant negative correlation during the winter.