The Portneuf River is a tributary of the Snake River.  This river runs thru the town of Lava Hots Springs.  There are several places in town to rent tubes ($3-$10) or you may bring your own.  You may rent by the hour or the day.  There are one, two, three and four man tubes.  You must bring your own Kayak.  Enter the river any time after the falls and exit the river just prior to the Center Street bridge.  Most rental places have maps if you need one.  It's about 2.5 miles at the longest point but you can do shorter.  A river walk runs along side this part of the river.  In late August the river is only about 2' deep and bigger people will "drag bottom" in a few spots.  The tube rental places will let you have a free life jacket with your rental.  The water in late August is very tepid, not cold at all.  Be sure to wear tie up shoes, not flip flops or slip ons or you will lose at least one of them on the way down.  This is a favorite activity when visiting Lava Hot Springs.  Kids as little as 3-4 can enjoy this river with an adult's supervision.