When you swing a fly on heavy fall water, you cast down and across, and the current pushes a downstream bow into your line. That bow is the belly, and it is what pulls the fly across the current and swims it broadside in front of a steelhead. A bigger belly drags the fly through the arc faster; lift the belly and set it upstream of the fly, and the fly slows down and sinks. Almost everything you do to control a swing in big water, the mend, the rod, the casting angle, and where you choose to stand, is a way of sizing that belly to the current in front of you.

A heavy fall rain brings the river up, speeds the current, and spreads it out, so the run you swung easily in September turns into a fast, off-color pull that throws a belly into your line the moment it lands. The faster water loads that belly bigger and quicker.