Effects of flow regulation and river channelization on sandbar bird nesting availability at the Lower Vistula River

Abstract

The Vistula transports large volumes of river load, with notable prevalence of bed load. Considerable flow irregularity over the course of a year creates favourable conditions for the formation of sand islands (bars) at the mid and lower river reach. As the bars remain emerged for long periods of time, they constitute resting and nesting grounds for water avifauna, particularly species of the Charadriidae and Laridae family. In consequence of regulatory works performed in the 19^th century at the lower Vistula reach, the channel has been devoid of natural islands overgrown with riparian forests and the accompanying minor sand bars. In turn I observe a prevalence of low and, as far as morphology is concerned, poorly diversified alternating accumulation forms. Moreover, operation of the dam erected in the 1970s has increased water level fluctuation frequency in the channel below, thus resulting in more frequent inundation of sandbars. This is particularly adverse for water birdlife, whose habitats become flooded during the breeding season. Morphometric research paired with the analysis of hydrological data allowed us to indicate threshold water flow conditions at which sandbars become submerged. Furthermore, efforts were made to determine the number of potentially successful breading seasons in the years 1986–2015. Identifying the dates and the range of flood waves caused by the dam operation made it possible to indicate the number of complete and interrupted breeding cycles on sandbars. Most importantly, the study seeks to answers the question whether the habitat conditions at the regulated lower Vistula reach are sufficient to allow water birdlife to bread.