An osprey glides above the river, while down on the water, a crane lifts piles of scrap metal from a barge, a conveyor takes gypsum to shore from a Maltese-flagged bulk carrier, and stacked shipping containers, topped by a full-sized tour bus, rise high above the deck of another barge bound soon for Alaska.

The osprey no doubt plucks fish from the river and carries them home to its young. Some humans do the same. They probably shouldn't. You want PCBs, pthalates, mercury? They're all here.

This is Seattle's industrial river, the Duwamish, It is soon to be the target of a $342-million Superfund cleanup, which will build on what have already been years of significant efforts to restore the river. Even with a Superfund cleanup, however, there are limits to what can be done, and to understand that, it helps to consider the natural history of the river, and its human history, continuing to the present day.

The polluted modern waterway is a far cry from the meandering river that Seattle pioneers found here in 1851, the one from which natives had harvested fish and clams for millennia. If you found a clam near the river mouth now, you'd be well advised to throw it back. The modern Duwamish has been shaped by the pursuit of profit.

Not even the water level is truly natural. Look at a wall of dark sediment rising from the water. A thin green line of vegetation parallels the river partway up. Port of Seattle environmental program manager George Blomberg explains that everything above the green line represents height added by fill, and everything below the green line represents the lowering of the river level by dredging. The green line itself is what's left of the original bank.

The river once carried nearly four times as much water as it does now. The Green River, which rises in the Cascades near Stampede Pass, met the White, which rose near Mount Rainier, then kept flowing north to what is now Tukwila. There, just past the tongue of land currently occupied by the Starfire soccer complex, the Green joined the Black, which drained Lake Washington at the current site of Renton. The Black and the Green combined to form the Duwamish, which meandered through a broad flood plain and then an estuary that stretched from the high ground in West Seattle to the high ground of Beacon Hill. The Port of Seattle estimates that originally, the estuary "included approximately 1,450 acres of intertidal sand and mud substrate, nearly 1,300 acres of intertidal marsh, and approximately 1,450 acres of tidal swamp (or forested wetland)."

Then, people started replumbing the watershed. After the White jumped its banks and joined the Puyallup during an early-20th-century flood, local farmers did enough earthwork to keep it there. When the Lake Washington Ship Canal was completed in 1916, the lake level dropped 9 feet, and Lake Washington water started flowing out through the canal, rather than through the Black. The Cedar River had already been diverted from the Black into the lake. With no water flowing in, the Black dried up.

Now, the Green just changes its name to Duwamish in Tukwila. Upstream, the Howard Hanson Dam — and a network of levees — keeps the Green from flooding Auburn and the rest of its former floodplain, and keeps salmon from swimming all the way upstream. Completed in 1961, the dam made possible the transformation of the Kent Valley from agriculture to warehousing and urban development.