I had concerns about that as well, but I found the following comment:

"Stream crossings: I found a pair of waders that weigh 9 ounces and fit easily into the side pocket of my pack. They easily fit OVER my hiking boots and take about 30 seconds to put on and 30 seconds to take off after the stream crossing.They are about crotch high when on. The cost is about $60 at Wiggys.com
Posted: Jun 03, 2010 tim"