Inner tubes are a lot like condoms–they’re both made of rubber, and should be chosen based on enjoyment, purpose and size. Before heading out to the river, you may want to try one on–the inner tube, that is. Be sure to purchase at least one spare tube especially for your beer cooler!
Inner tubing is the poor man’s rafting. More importantly, it’s an activity accessible even by the most inept of athletes. The basic instructions are as follows: 1) Sit in the tube–once it’s in the water, dummy. 2) Let the river’s current take care of the rest.
Some of the state’s best downriver floats are within 20 minutes of Portland. On the Sandy River, start at Dodge Park. Exit 12 from I-205, hwy 212 through Boring to Ten Cyck Road; follow signs to park. (Also look for the rope swing!) Get out at Lewis & Clark State Park.
To return to your launching spot, you will either need to hitchhike back, or pre-plan and park a car where you plan to leave the river. (Remember, park the second car DOWNriver, as you will float with the current. You wouldn’t believe how many people screw this up.)
Warning: After five beers, swimming skills deteriorate. Remember, drowning is both embarrassing and a bummer.

Inner tubes are available at Popina Swim in NE Portland. The most kick ass inner tubes ever created. Beverage holders, removeable cooler and back rest.
http://www.popinaswimwear.com/retro_suit_b…
UPDATE:
Popina has expanded out inner tube offerings July 2010!
Popina now offers FOUR tubing options:
1) The basic River Rat inner tube for $15
2) Middle of the road River Run wth backrest and beverage holders for $30
3) Deluxe Double tube with back rests, beverage holders and cooler $56
4) Deluxe single the Sun Seeker backrest, beverage holders and cooler $57
PLUS
Floating beverage cooler – $30
Car adaptable electric hand pump – $15
Manual double action hand pump – works great – $10
Tube the Clackamas River, Sandy River or Wilson River NOW!
Man I love tubing
http://www.popinaswimwear.com/retro_suit_b…
Dodge Park to Lewis and Clark Park would take a ridiculous long time to tube.
Try starting at Dabney or Oxbow, if you have time to waste.
how long does it take to float from dodge park to lewis and clark? says its 12.7 miles.