Suspended sediment and total phosphorus transport in a major Danish river: methods and estimation of the effects of a coming major restoration

Abstract

1. The planned restoration of the lowermost 18 km of the Skjern river system (catchment area 2490 km2) through re-meandering the river to its former course and the creation of a shallow lake and ponds is the largest river restoration project in Europe. An important aspect of the project planning and design has been to measure suspended sediment (SS) and total phosphorus (TP) transport in the project area, and to assess the inter-annual variation. 2. SS and TP concentrations were measured continuously (every fourth hour) from 1993 to 1995 in the River Skjern and its main tributary, the River Omme, using automatic sampling equipment (ISCO). In addition, discrete samples were collected monthly in the remaining five smaller tributaries. Estimated SS transport in the Skjern river system in 1994 and 1995 determined on the basis of continuous sampling was approximately 60% greater than that determined on the basis of discrete sampling. Empirical models for SS and TP transport were developed based on the data collected in this study and applied to a 31-year time series of daily discharge values. Mean annual transport amounted to 12 220 t SS and 100 t TP corresponding to 5 t SS km−2 yr−1 and 41 kg TP km−2 yr−1, respectively. 3. Assessment of the effects of the planned restoration project, based on measured transport and estimated SS and TP retention rates for different areas of the lower river system, revealed that SS and TP transport in the river will be reduced by 37% and 20%, respectively. Restoration will therefore considerably enhance the natural self-purification capacity of the river system. In addition, restoration will reduce nitrogen and ochre loading of Ringkjøbing Fjord, thereby improving environmental conditions, and re-meandering will improve habitat quality and diversity in the river system. The study stresses the importance of considering streams and riparian areas as an entity when evaluating the effects of restoration activities. ©1997 John Wiley & Sons, Ltd.