The Tualatin River is a small to medium sized river in the State of Oregon, and is one of the major tributaries to the Willamette River.
The Willamette River runs through the Willamette Valley, which is a wide basin, on the order of 100 kilometers, between two ranges: the Cascade Mountains, and the Coast Range. The Coast Range is much less tall than the Cascades, and has no glaciers, and has a snow pack that melts relatively earlier in the year. In the part of the Willamette Valley that the river runs through---Washington County, the land is fairly flat, so the river meanders quite a bit. Although Washington County is a somewhat posh area, and is sometimes mocked for its suburban sprawl, it also has much of its natural wetlands preserved. Because of these reasons, the Tualatin has a much mellower profile than most Western rivers, rising in relatively lower hills, and then meandering over a large amount of flat ground, picking up various other low, slow tributaries, before finally emptying into the Willamette. Because the West Hills separate the larger Willamette Valley from the Tualatin's basin, it has to go south and meet the Willamette in a gap in the hills.
All of this is natural behavior for a river to take; but the effect might be less photogenic than the rocky-banked, white water rivers that come crashing down from the Cascades. The Tualatin River is very opaque, showing a muddy brown or muddy green color. I visited the mouth of the river today and found that the rocks at its mouth were covered with several inches of thick green algae, and almost as richly covered with snails. Some of this growth is probably due to the fertilizer-rich farmland the river goes through, but some of it is probably just the natural richness of a river in a wet, mild climate.
I don't actually know if the river is more or less polluted than other rivers in Oregon. I don't even know if such information could be available. Because it passes through some fairly intensive industrial, residential and agricultural land, I have to guess that pollution is a problem. However,the river system also has lots of wetlands that hopefully help filter out the pollution. If the watershed of the river doesn't get recklessly developed in the coming years and decades, the river's muddy green glory will be around for future generations to enjoy.