"DICK," SAID I, "I think I have this thing figured out." Dick Blalock smiled from the stern of the canoe, where he was covetously eyeing his last hummus- on-pita-bread sandwich, and waved me on.

"See, when the water's this low and clear, the bass get spooky. If you cast your lure close to the boat, they come out and take a look, but when they see us they won't take.

"I believe we should be using fairly heavy lures and light line so we can cast a long way. That way the lure lands in virgin water."

Blalock weighed this hypothesis as we drifted along. While he pondered, the small Rapala lure attached to his spinning outfit trailed about three feet behind the boat. I was afraid it might snag on something until a medium-sized smallmouth bass lurched out of the depths and gobbled it.

"So much for that theory," laughed Blalock.

Fishing wouldn't be much if it didn't surprise you. For folks like Blalock and me, it's a surprise a minute. "They say 5 per cent of the fishermen catch 95 per cent of the fish," he said as we set out on an afternoon float down the Potomac last week. "I pride myself on being among the 95 per cent that catch 5 per cent of the fish."

But Blalock, a retired Foreign Service officer with a robust appetite for Middle Eastern food and fly fishing, said that despite his skill deficiencies he's still managed to catch (and release) more than a thousand smallmouth bass this summer from his favorite stretch of Potomac, the 31/2 miles just above the U.S. 15 bridge at Point of Rocks, Md., 45 miles from Washington.

Blalock hits the stretch about twice a week from spring to fall and very rarely has a bad day, but he and most other float fishermen would agree that no season is quite as pleasant and productive for bass fishing as autumn.

For one thing, the water clears up as it cools, algae blooms being at their worst in warm water. Clear water presents challenges for the angler, but it's worth it for the sense of well-being that accompanies fishing in places where you can see the bottom.

The Potomac was just about crystal clear and 60 degrees last week, which is close to ideal temperature to spur bass to feed in preparation for the winter. Gerald Almy, who lives within casting distance of the fine smallmouth waters of the North Fork of the Shenandoah, says river fishing stays good until the water drops into the mid to lower 50s, and he said the larger bass are most active as temperatures decline.

Almy believes bigger rivers like the Potomac are best for fall fishing because they offer pockets of deep water where bass congregate. Blalock and I found that to be true as we scored best whenever we crossed over some riffles into a deep pool below.

How do you fish for fall smallmouth? My method, with flies, lures and even with bait, is to fish across the current, casting as far as possible and then retrieving slowly as the lure gets washed downstream.

Almy said bass are particularly susceptible in the fall to surface lures like Tiny Torpedoes, Lucky 13s and Creek Chub Darters, as well as surface poppers fished on fly gear. This is great fun, since you get to see the fish strike, but in truth I've found nothing that produces results as consistently as a plain old squirrel-tail Mepps spinner.

An undeniable pleasure of stream fishing is reading the water. Bass hang wherever they can sneak up on unsuspecting bait. They hide behind rocks, next to sunken trees and at the foot of rapids or riffles, where food is likely to be washed over. It's nice to have polaroid glasses to cut the glare and see deep, but even without them, if you study the way the water moves, you can identify, with practice, where submerged obstacles lie. And it's a satisfaction to pick a spot where a bass ought to be, cast to it, and get a strike.

A boat of some sort expands the range. Blalock uses a square-stern canoe. But you can walk upstream from the U.S. 15 bridge, or along the Potomac banks just about anywhere above Great Falls, and find good smallmouth water.

You'd think with superb fishing so close by the river would be jammed, but last week Blalock and I saw exactly one other boat in six hours.

I think I know why that is, but I'm going to keep it to myself.

I'm tired of going out on a limb.