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Here's a slideshow from Portland's Columbia Slough Trail, which runs along the north shore levee of the Columbia Slough between N. Portland Road and Vancouver Avenue. I walked the length of the trail (minus a bit at the west end) one summer day, primarily as a way to get photos of a lot of the bridges over the slough, as well as the two islands in the middle of the slough that are technically Portland city parks (albeit inaccessible ones). After taking the trail I figured it merited a post of its own too. It's not precisely a nature trail, though; as it follows along the slough the trail passes the Heron Lakes golf course, Portland International Raceway, Portland Meadows (Oregon's sole surviving horse racing venue), and assorted industrial stuff. I dunno, I thought that was kind of interesting, or at least unusual. Your mileage may vary, obviously. (For what it's worth, it turns out that it's considered bad form to heckle the golfers, no matter how terrible they are, or how ugly their golf clothes look. You learn something every day, I guess.)
The stretch of trail between Portland Road & Denver Ave. is the original stretch of the trail, completed back in 2001, and planned back in the late 1990s. It connects with the north-south Peninsula Crossing Trail at the Inverness Force Main Bridge, the pedestrian bridge that's actually a bridge for a cleverly concealed giant sewer main. The original stretch of trail hasn't gotten proper upkeep in recent years and has developed potholes, which is probably not something you want on the top of your levee. More recently, the mile-long stretch between Denver & Vancouver Avenues just opened in January 2014, so it's in great shape, and the shiny new Vancouver Ave. bridge is designed to be bike friendly, for a change. Maybe the rest of the trail will get more attention now that it's becoming a through bike route and not just a weird disconnected stretch of trail in the middle of nowhere.
The bit between the old and new segments at Denver Ave. isn't so great right now. It's a busy street and your best bet is to take the Schmeer Rd. underpass under the street, and even then there isn't a sidewalk or bike lane, and you have to walk on the shoulder and look around for trouble. The city and ODOT want to redesign the intersection in the near-ish future, since the current intersection isn't great for cars and trucks either. The plan is to move the Schmeer Rd. intersection further north, with the current underpass becoming part of the trail.
The long term plan (or sorta-plan) is to eventually have a trail along the entire length of the slough, from Kelley Point Park all the way out to roughly Troutdale. I'm not sure about the eastern portion, but the stretch between the Willamette and the Peninsula Drainage Canal (near NE 33rd) should be doable, since it would involve building on top of existing levees, which are already publicly owned. Publicly owned & operated by the county's four obscure and seriously underfunded drainage districts, to be exact. But that's a whole other story.