Using large wood to restore streams in Central Europe: potential use and likely effects

The potentials for the use of large wood (LW) in stream restoration projects were quantified for streams in Central Europe (total stream length assessed 44,880 km). Two different restoration methods were investigated: recruitment (passively allowing natural LW input) and placement (active introduction of large wood pieces into streams). The feasibility and potential effects of each method were studied for three different scenarios, according to the land-uses to be permitted on the floodplain: (a) only natural-non woody vegetation, forest, and fallow land occur on the floodplain, (b) including pasture and meadow, (c) including pasture, meadow, and cropland. Hydromorphological data were used to identify stream sections where LW recruitment or placement are feasible, and the likely effects of both restoration methods on channel hydromorphology were predicted. Passive recruitment is feasible for only a small percentage of the total channel length in the study area (similar to 1% for all three scenarios). Active placement of LW can be used in much higher extent: 6.5% if only natural non-woody vegetation, forest, and fallow land can occur on the floodplain, 20.2% if stream segments bordered by pasture and meadow are included, and 32% if cropland is included in addition. There are differences between (1) the lower-mountainous area, where a large number of channel segments can be restored yielding an improvement from a moderate/good to a good/excellent morphological status and (2) the lowlands, where only a small number of channel segments can be restored yielding an improvement from a bad to a moderate morphological state. The latter upgrading might be sufficient to reach a 'good ecological status' as defined by the EU Water Framework Directive. The results of this study show the suitability of large wood recruitment and placement as appropriate methods to markedly improve the hydromorphological state of a large proportion of the streams in the study area.