The Potomac had almost carried his dog Princess into the downriver darkness. Now it had Warren Helm knee-deep and off balance in its wild current as he stooped to free his homemade net of chicken wire and twine. In the soft light cast by the lamps on Chain Bridge above and a bonfire on the shore behind him, Helm, 35, looked like a Mark Twain character about to be swept into an adventure.

"I've been coming down here for the herring run since I was 5 years old," said Helm, a government worker who was sharing a rocky stretch of Virginia shore with 50 other men, women and children one night last week. "This is like getting free fish at the supermarket."

When the herring run up the Potomac each spring in great silver schools to lay their eggs, the water above Key Bridge boils with jumping fish and frenetic fishermen, confronted with a catch of biblical proportions.

The herring swim with each incoming tide up the Potomac as far as Little Falls dam. Waiting to ambush them along the way with nets and snag hooks are an army of people, some of them sportsmen, some of them unemployed and hungry. For both groups, the return on time invested can be tremendous.

"I've been here days when you could fill 15 five-gallon buckets in a matter of minutes," said Jim Roberson of Lovettsville, Va., a distributor for an oil company who never crosses Chain Bridge in April without looking at the river below for signs of the silver leapers.

Downstream at Fletcher's Boathouse there is no need to look. When the herring empty their egg sacs by jumping from the water to slap their tails against the surface, the sound is loud enough to wake the old men who doze on benches in the sun and lure younger ones like Dick Teehan to the fast moving current.

"Yesterday there were so many herring out there it sounded like raindrops," said Teehan, watching a dozen red rowboats on the river.

There are many ways to catch herring. But when they are thick as sardines on the tide, the easiest method is to "dip" for them with long-handled nets. And nowhere is "dipping" more popular than just below Chain Bridge.

"A few days ago some people took out as much as 1,000 pounds of fish in four hours from here," said James Nash, an out of work community planner who was selling hooks, nets and cold sodas last week from the trunk of his car on the Virginia side of the bridge where access to the river is easiest. "I've talked to a lot of people who have been coming here for more than 20 years."

For a week, there have been herring hunters beside the Potomac 24 hours a day. Drive over Chain Bridge after dark and you will see their fires on shore. Return at dawn and smoke is still rising from what appears to be a hobo camp.

The worst aspect of that scene is the trash left by a few slob anglers, who casually toss beer bottles and cans into the river. The best part is the camaraderie among as disparate a group of fishing folk as ever shared a shore. You might see, for example, a Vietnamese fisherman lend his knife to a West Virginia truck driver who will show his gratitude by offering a chaw of tobacco.

Stan Chruz, 23, was delivering a piece of heavy equipment from McLean to Washington last week when he looked below Chain Bridge and saw a sight to make his fish-loving heart stop.

"It was incredible. There were 10 fish jumping every second," said Chruz who brought his fishing rod and fishing buddy, Bruce Dressel, to the spot the following day. They missed the tide, but still caught fish, including a 10-pound carp that Dressel snagged with a treble hook.

Fifty yards downstream, Robert Leibner, 29, a Washington attorney, was casting plain, gold hooks into the current on the advice of a friend who insisted that herring would bite the baitless hooks.

"It sounds impossible, but it works," said Leibner after reeling in three herring and one white perch.

Mike Mock and his 16-year-old friend Chris DeLashmutt had walked from their homes in Arlington to the river just to watch the fishing show. But after finding an old snag hook and 30 yards of fishing line on the shore, they decided to join the circus.

"If I catch anything with this outfit I deserve something," said Mock, as he tossed his hook into the water, then jerked it back towards him with the hand-held line. He snagged a fish on his first try.

For ingenuity, however, Warren Helm won the prize. With a piece of chicken wire, a few green tree limbs and some twine, he fashioned a handsome herring net. He gave credit for the engineering feat to his father.

"My father showed us what was going on here. We used to make a migration, the family and the neighborhood kids, down here from Virginia." This year Helm brought his own 13-year-old son, one of his playmates and Princess, a 3-year-old German shepherd, to the river. "Now it's my responsibility," said Helm.