Far from being the serene, natural streams of yore, modern rivers have been diverted, dammed, dumped in, and dried up, all in efforts to harness their power for human needs. But these rivers have also undergone environmental change. The old adage says you can't step in the same river twice, and Ellen Wohl would agree-natural and synthetic change are so rapid on the world's great waterways that rivers are transforming and disappearing right before our eyes.            A World of Rivers explores the confluence of human and environmental change on ten of the great rivers of the world. Ranging from the Murray-Darling in Australia and the Yellow River in China to Central Europe's Danube and the United States' Mississippi, the book journeys down the most important rivers in all corners of the globe. Wohl shows us how pollution, such as in the Ganges and in the Ob of Siberia, has affected biodiversity in the water. But rivers are also resilient, and Wohl stresses the importance of conservation and restoration to help reverse the effects of human carelessness and hubris.            What all these diverse rivers share is a critical role in shaping surrounding landscapes and biological communities, and Wohl's book ultimately makes a strong case for the need to steward positive change in the world's great rivers.