10 years after the largest river restoration project in Northern Europe: Hydromorphological changes on multiple scales in River Skjern

Abstract

a b s t r a c tThe lower river Skjern (Denmark) historically contained a large variation in habitats and the river ran through large areas with wetlands, many backwaters, islands and oxbow lakes. During the 1960s the river was channelized and the wetland drained. A restoration during 2001-2002 transformed 19 km of channelized river into 26 km meandering river. The short-term effects of this restoration have previously been reported and for this study we revisited the river and with new data evaluated the long-term (10 years) hydrological effects of the restoration. The evaluation was done on three different scales: (1) instream habitats, (2) channel stability and (3) re-connection with the floodplain. In-stream habitats had changed little over the past 10 years and the habitats today showed close similarity with the habitats recorded immediately after the restoration. Measurements of channel stability showed that erosion and sedimentation have changed the cross-sectional profiles over the last 10 years, resulting in a net input of sediment to the lower reaches of the river. However, the change of channel form was a slow process and predicted bank retreat over a 100 year period was only up to 6.8 m. Hence the formation of lost habitats (islands, backwaters and oxbow lakes) is a very slow process and the spontaneous development of these habitats will take centuries. Furthermore, the evaluation also showed that the restoration re-connected the river with its floodplain and large areas of riparian areas are today periodically flooded, but that the flooding is controlled and tamed due to the restoration design. The restoration of River Skjern has therefore failed to re-create the natural habitats formerly present and the natural dynamic processes that shape these habitats are slow. To speed up this process we therefore recommend restoration engineering using a natural guiding image when restoring lowland rivers in the future and through this restoring the lost habitats and the dynamic processes characteristic of natural rivers.

Contrasting the roles of section length and instream habitat enhancement for river restoration success: a field study of 20 European restoration projects

“…In the cases of the Skjern (compare Kristensen et al . ) and the Narew, however, the restoration measures mainly affected the floodplains, in which large flood prone areas were created, while there were relatively few changes in stream bottom substrates. In contrast, in the short restored section, Stora (Denmark), the focus was on instream measures, which directly generated habitats for aquatic organisms.…”

Long‐term performance of in‐stream restoration measures in boreal streams

“…Lack of spawning gravel may restrict the recovery of salmonid populations (Palm et al, 2007), although the effects of gravel addition on fish populations are not always obvious (Koljonen et al, 2012). Therefore, restoration of natural processes that create and maintain habitats for the stream biota is more relevant than persistence of individual instream structures (Roni et al, 2008;Kristensen et al, 2013). Assessment on the current performance of restoration measures, and their compliance with the initial restoration design, showed that most restoration structures were functioning reasonably well.…”

“…However, dynamic ecosystems are subject to continuous change, and, consequently, project outcomes are likely to vary over time (Lake et al, 2007). Therefore, restoration of natural processes that create and maintain habitats for the stream biota is more relevant than persistence of individual instream structures (Roni et al, 2008;Kristensen et al, 2013). Ecological restoration should allow movement of channel features, instead of constructing fixed structures that resist natural channel evolution and delay the recovery of ecological integrity Gillilan et al, 2005).…”

Changes in Vascular Flora of the Rzeszow Reservoir after 20 Years (SE Poland)

“…Floristic research may be used to determine environmental conditions and to monitor changes [16,64]. Plant species -because of their diversity, life forms, geographic distribution, and ecology -are useful tools for research on ecosystems [64,67]. The flora naturalness index is a measure of transformations, with high…”