Starting at Lake Tear of the Clouds, a two-acre (0.8-ha) pond in New York's Adirondack Mountains, the Hudson River runs 315 miles (507 km) to the Battery on Manhattan Island's southern tip, where it meets the Atlantic Ocean. Although polluted and extensively dammed for hydroelectric power, the river still contains a wealth of aquatic species, including massive sea sturgeon (Acipenser oxyrhynchus) and short-nosed sturgeon (A. brevirostrum). The upper Hudson is fast-flowing trout stream, but below the Adirondack Forest Preserve, pollution from municipal sources, paper companies, and industries degrades the water. Stretches of the upper Hudson contain so-called warm water fish, including northern pike (Esox lucius), chain pickerel (E. niger), smallmouth bass (Micropterus dolomieui), and largemouth bass (M. salmoides). These latter two fish swam into the Hudson through the Lake Erie and Lake Champlain canals, which were completed in the early nineteenth century.

The Catskill Mountains dominate the mid-Hudson...