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B. Baur and S. Schmidlin
al. 2002). We review major changes in the biota of the river Rhine, focusing
on mechanisms underlying changes in species abundance following the
invasion of non-native species. Our emphasis is on benthic macroinvertebrates
but interactions with other animals are also considered. Along the
way, we identify important gaps in knowledge and suggest areas for further
research.
15.2 The River Rhine
With a length of 1,320 km and a catchment area of 185,000 km 2 , the river
Rhine is one of the largest rivers in central Europe (Van Urk 1984; Friedrich
and Müller 1984). It originates in the Eastern Swiss Alps, flows north to form
the frontier with Liechtenstein and Austria (Alpenrhein), and empties into
Lake Constance (Fig. 15.1). The Rhine (High Rhine) then re-emerges and
flows west, mainly on the border between Switzerland and Germany. In
Basel, it turns to the north and forms the southern part of the border
between France and Germany (Upper Rhine) in a wide valley, before entering
Germany exclusively (Middle Rhine). Here, the Rhine encounters some
of its main tributaries (the Neckar, the Main and then the Moselle). Between
Bingen and Bonn, the Rhine flows through the Rhine gorge, a formation created
by erosion (this gorge is a UNESCO World Heritage Site since 2002).
After passing the Ruhr area, the Rhine (Lower Rhine) turns west into The
Netherlands. After crossing the border, it splits into three main distributaries,
the Waal, the IJssel and the Nederrijn/Lek, before discharging into the
North Sea.
The flow regime can be characterized as rain-fed/snow-fed, the highest
water levels usually being attained in March–May and the lowest in August–
November. The mean annual river discharge of the Rhine is 1,032 m 3 s –1 in
Basel and 2,260 m 3 s –1 (range 800–12,000 m 3 s –1 ) at the Dutch border. This
results in the minimum and maximum water levels differing by up to 8 m in
The Netherlands (Van Geest et al. 2005).
The deterioration of the Rhine started in the Middle Ages, with the deforestation
of large areas on the floodplains (Nienhuis and Leuven 1998). By the
early 18th century, almost all beech and oak forests had been replaced by
grassland. The river morphology became increasingly degraded because of
straightening, reduction of channel networks to a single channel, and disconnection
from the floodplain. In the 19th century, major river regulations in the
Upper and Lower Rhine modified the river bed. For example, in the so-called
Tulla-correction carried out between 1817 and 1874 and also in subsequent
channelisations, the Upper Rhine north of Basel was transformed from a river
system up to 6 km wide, with numerous branches, slow-flowing meanders,
islands, and sand and gravel flats, into a 130-m-wide, fast-flowing sealed canal