Unbuckling the wasit belt and sternum strap on a pack is the official method taught at NOLS. Losing a pack is better than losing your life. When you fall in a river with a significant current, you need to escape from your pack immediately. A pack, with all its little loops and etc can easily get caught on a branch or rock on the stream bottom and hold you under water.

In cases where my pack is really heavy, I have ferried two or more loads across a river, rather than take a chance on a heavy pack with my wasitbelt hooked.

The whole purpose of lightweight backpacking is to have a relatively light pack - and if it is so heavy or ill balanced that you need the wasit belt for stability, I would suggest that it is too heavy or needs re-packing.

Streams are not the only hazard for your pack. A friend of mine was climbing the east face of Mt. Whitney and set her pack down at a rest break and it rolled off the mountain, never to be found again! And she was a very experienced climber. Everyone makes mistakes.