Big-time saltwater fishermen know that the richest fishing grounds off the Eastern United States lie in the Gulf Stream, a natural warm river within a cold sea.

Small-time freshwater fishermen from Washington, D.C., have a little Gulf Stream all their own -- a man-made ribbon of warm water that runs south from the Pepco power plant at Dickerson, Md.

The Potomac's Gulf Stream doesn't show up on any maps or charts. To find it you have to buckle up and go searching. But the bass bite there all year long, and when a neighbor turned up with a midwinter stringer of smallmouths up to three and four pounds last week I knew it was my turn to go exploring.

Information on a hot February fishing hole is not exactly easy to extract. I had an ally in Glenn, an old bassfishing partner. At first our source was giddy from his success and eager to share. But the more we pressed, the tighter-lipped he grew.

Finally he clammed up altogether. We were on our own.

Our assets: A nub of information, an old tin boat with a superannuated outboard and a day with a warm south wind. It could have been worse.

There are two put-ins downstream from the Pepco plant, where the water emerges warm after it's used to cool the electric generators. One launch site is at Edward's Ferry, which isn't a ferry any more but just a muddy spot where the road ends at the river. The other is White's Ferry, where the ferryboat Gen. Jubal Early still hauls people and cars between the Maryland and Virginia shores.

We tried Edward's first, but after dipping a hand in the water decided that it couldn't be any colder unless it was solid. We moved upstream, closer to the warm-water discharge.

White's Ferry water temperature wasn't much better but there was a fisherman there, seining minnows near the bank. He said he'd caught his limit of bass every day for the last week, fishing upstream toward the power plant behind some islands.

He zoomed off in his Boston Whaler and before too long we had launched and were poking along behind in the overloaded tin boat.

This is the Potomac's homeliest season. In another six weeks the banks will be decked iin spring green but today it's a cold, dank, brown and desolate place. We zipped up our flotation jackets, knowing that a capsize would involve extraordinary danger in the lonely, wide stretches.

The Whaler man had ducked in behind the first island on the Maryland shore, but by the time we reached there he had moved elsewhere, not to be seen again. Along the banks shore anglers were filling in, fishing the bottom for catfish and carp and doing well.

We could not find the bass.

We couldn't even find a place where the water seemed any warmer than the midstream current. Two, three and four hours passed. We studiously worked all the shorelines, casting diving plugs and rubber worms on drop-offs and around fallen trees, waiting for the first bump from the first bass of the new season.

Nothing.

"Let's see if we can make it up to the power plant. We know the water's warm there," Glenn said. I put the little motor on full bore and headed up the middle of the river. Glenn knelt in the bow and directed me with hand signals around big boulders.We could watch the bottom sweep by four and five feed down.

A couple miles upstream the water ran faster and shallower. When the motor finally bumped bottom we trimmed it up and drifted back downstream with the current until a little cut opened on the Maryland side. We slid in there and followed a creek back behind an island. The water opened up into a wide slough.

"Wow," Glenn said. "I think this is it."

He pointed to the Maryland shore.

The water along the bank was, well, different. There was a band about 30 feet wide where it took on a murky color, unlike the clear water of the main stream. We paddled over and dipped a hand in. Warm. Probably 10 degrees warmer than the rest.

We began casting the shoreline with renewed vigor, drifting the little boat along with the current and anchoring at likely-looking spots. In an hour we managed to catch five bass, not particularly big ones but bass nonetheless. Then a warm rain erupted and sent us scurrying back to the landing.

Dave Meeker of Frederick was waiting at the ferry when we pulled in, drenched. He'd just quit, too.

"How'd you guys do?" he asked. We told him about our little fish, which we'd turned loose. He reached in his boat and yanked out a stringer of keepers, including a three-pounder.

Meeker said he'd thrown back a dozen other smaller bass and he'd caught them all in 2 1/2 hours, mostly on small crankbaits.

Of course, he had some spots. Now we do, too.

There are some who think the Potomac is the best smallmouth river anywhere.

For certain, big smallmouths are more common in Washington's river upstream from Great Falls than any other river I've seen.

But fishing the river is always an iffy proposition. Last spring rain and melting snow rendered the Potomac unfishable practically until May.

This year, so far, things look better. There's very little snow in the mountains. Rains have been gentle.

Early signs point to a good spring. But why wait for spring when the Potomac's Gulf Stream is already jumping?