One of those plants everyone should learn to recognize is poison ivy. As the temperatures heat up (if?) people's thoughts turn more to cooling off at the river; but along the streams and rivers of this area is the perfect habitat for poison ivy- more specifically western poison ivy, Toxicodendron rydbergii (Rhus radicans).
At the base of the cottonwood above is a clump of western poison ivy- right along an infrequently used trail. Though most animals are immune- 85 to 90% of people are allergic to the oil that this plant produces.
The chemical irritant poison ivy makes is called urushiol, and can induce anything from a mild rash to a full blown allergic reaction requiring a trip to the hospital and a steroid injection depending on how allergic you are and how much of the stuff you get on you.
The best thing to do is to learn to identify the plant so you can avoid it. If you do suspect you've walked through it, wash any affected parts immediately. Apparently dish soap works better than regular soap as it is designed to cut through grease, and there is also specific washes like Technu which work really well. Wash all clothes with even a trace of oil on them thoroughly. Sometimes all it takes is secondary contact with something that has the oil on it- like a pants leg, or other leaves along a trail to start you itching. The oil can also rub off your pet's fur onto you, so don't let Fido loose next to the river if you plan on petting them in the future (also by the river are a plethora of ticks, so not a good idea anyway).
Though I've heard it called poison oak by many people, the particular species we have is poison ivy- not poison oak- though both species aren't really an oak or an ivy, those are in totally different plant families than Toxicodendron- they are just commonly called that. The family western poison oak and poison ivy is in, Anacardiaceae, includes our native sumac, Rhus glabrum and also cashews and pistachios- though these species don't produce irritating oils.
I always see western poison ivy next to rivers, there are some good patches of it along the Columbia, as well as all along the Wenatchee River. It also likes seasonally moist spots in the desert, along little creeks or in draws, moist valley bottoms in the dry forest, and seems to enjoy areas around the climbing rocks out the Icicle and Tumwater canyons too. Not a good way to start or end an excellent day of climbing, so keep your eyes open for plants with three leaves. Sometimes the leaves are lobed, and sometimes not. The plants can be six inches tall or six feet tall depending on conditions, and are rhizomatious- meaning they spread by specialized roots which can form large colonies of the dreaded weed.
More on identifying poison ivy in its various states and pictures of what damage the allergic reaction causes- What Poison Ivy Looks Like
The Toxicodendron species of the US and distribution from the USDA Plants Database
Information on Technu, available at pharmacies and outdoor gear stores- well worth the money if you are ever exposed.