UPPER HUDSON RIVER ESTUARY (USA) FLOODPLAIN CHANGE OVER THE 20TH CENTURY

Detailed surveys of the upper Hudson River Estuary and its floodplain from the early 1900s and digital mapping of the same areas today provide an opportunity to evaluate changes over the 20th century. This study uses a geographic information system to quantitatively compare water areas and islands mapped by the United States Army Corps of Engineers in 1907 and 1911 along an approximately 60‐km reach from Athens to Troy, NY, with the same features mapped in the late 20th century. The comparison shows a substantial decrease in total water area approximately 30% less than the 1907–1911 quantity, with secondary channels disproportionally affected (~70% less). The number and total area of islands has also dramatically decreased by approximately 65% and 85%, respectively. These changes primarily reflect the success of navigation improvement projects undertaken since the 19th century that transformed a shallow, island‐braided river in the study reach to one characterized by a deeper, single‐thread channel. Dredge spoils from the main channel were used to fill secondary channels and other backwater areas, a practice with implications for reproduction, growth and/or survival of native plants and animals. Published in 2011 by John Wiley & Sons, Ltd.     

HISTORIC CHANGES (1941–2008) IN SIDE CHANNEL AND BACKWATER HABITATS ON AN UNCHANNELIZED REACH OF THE MISSOURI RIVER

Flow regulation has had pervasive effects on aquatic ecosystems within the world's large rivers. While channelization on the lower Missouri River has led to major changes in the river and its floodplain, including the loss of shallow water habitats, effects of upstream dams on unchannelized reaches on the Missouri have not been formally assessed. We quantified changes in the number and size of off‐channel habitats, specifically backwaters and side channels, on the 95‐km unchannelized reach of the Missouri below Gavins Point Dam (Yankton, South Dakota) using historical (1941, 1983–1985, 2008) aerial imagery. Total and mean areas of side channels declined by 77% and 37% and total and mean length decreased by 79% and 42% from 1941 to 2008. Total area of backwaters increased by 40% from 1941 to 2008, whereas mean area decreased by 36%. Our findings suggest that sharp declines in the area and length of side channels have occurred on this unchannelized remnant reach of the Missouri River, with likely significant impacts on aquatic ecosystem processes. Copyright © 2011 John Wiley & Sons, Ltd.     

EUROPEAN SILVER EEL MIGRATION AND FISHERIES‐INDUCED MORTALITY IN THE HAVEL RIVER SYSTEM (GERMANY)

By using acoustic telemetry, the behaviour and fishery mortality of silver eels Anguilla anguilla (L.) were studied during the early phase of the spawning migration through the Havel River downstream to the river Elbe. A total of 99 female silver eels were implanted with acoustic transmitters and released at two locations in the Havel River system in autumn 2007. Tag recaptures by fisheries were used to assess fishery mortality. Most eels continued their downstream migration immediately after release during October–November in the year of release. However, some eels continued their downstream migration the following spring or autumn; only a few eels migrated in winter and summer. Tagged eels reached the river Elbe from 2 days to more than 1 year after tagging. Fishing efficiency was very high, and by the end of December 2008, only 8% of the tagged eels had reached the river Elbe, whereas 25% were reported caught by commercial fishers and 2% by recreational fishers. The remaining 65% of the tagged eels were unaccounted for at the end of the study period. Copyright © 2011 John Wiley & Sons, Ltd.     

POST‐DAMMING FLOW REGIME DEVELOPMENT IN A LARGE LOWLAND RIVER (VOLGA,RUSSIAN FEDERATION): IMPLICATIONS FOR FLOODPLAIN INUNDATION AND FISHERIES

Periodic flooding plays a key role in the ecology of floodplain rivers. Damming of such rivers can disturb flooding patterns and have a negative impact on commercial fish yield. The Volga River, the largest river in Europe, has a regulated flow regime after completion of a cascade of dams. Here, we study effects of damming on long‐term discharge variability and flood pulse characteristics. In addition, we evaluate the effects of the altered flood pulse on floodplain ecosystem functioning and commercial fish yields. Our results indicate that both flood pulse and fish populations of the Volga–Akhtuba floodplain have varied considerably over the past decades. After damming, annual maximum peak discharges have decreased, minimum discharges increased, but average discharges remained similar to pre‐damming conditions. Moreover, because of bed level incision of over 1.5 m, a higher discharge is needed to reach bankfull level and inundate the floodplains. Despite this significantly altered hydrological regime and subsequent morphological changes, current discharge management still provides significant spring flooding. However, commercial fish catches did decrease after damming, both in the main channel and in the floodplain lakes. All catches were dominated by species with a eurytopic flow preference, although catches from the main channel contained more rheophilic species, and floodplain catches contained more limnophilic and phytophilic species. The strong increase of opportunistic gibel carp (Carassius gibelio) around 1985 was apparent in the main channel and the floodplain lakes. Despite the hydrological changes, the decrease in overall catches, and the upsurge of gibel, we found a strong positive effect of flood magnitude in the previous year on commercial fish yield in the floodplain lakes. This suggests that under the current discharge management there still is an increased fish growth and/or survival during high floods and that functioning of the floodplain is at least partly intact. Copyright © 2011 John Wiley & Sons, Ltd.