For 10 points, name this central European river, the second-longest in Europe.
Ukraine is controversially planning to reopen the Bystroe Channel by digging out wetlands in this river's delta. This river passes through the Devin Gate and, downstream, the Iron Gate gorge on the Romanian-Serbian border. This river, which empties into the (*) Black Sea, passes through Vienna, Bratislava, Budapest, and Belgrade--the most capital cities of any river. For 10 points, name this central European river, the second-longest in Europe.
Ukraine is controversially planning to reopen the Bystroe Channel by digging out wetlands in this river's delta.
Danube River