While biking along the C&O Canal towpath near the Seneca Creek Aqueduct last fall, Alan Whelihan, 66, stopped to gaze at the rushing water of the Potomac. As he got back on his bicycle, his pants leg snagged on the seat and sent him catapulting down an embankment.

"I saw him fly through the air and fall eight feet down onto the concrete," recalled his wife, Joan, 63. "I thought he'd broken his neck."

A passing bicyclist also saw the accident and used a cell phone to call paramedics. Alan Whelihan suffered three broken ribs and a broken shoulder and was temporarily blinded.

"We'd have been in real trouble if someone didn't have a phone," Joan Whelihan said as she and her husband, of Potomac, walked along the towpath last week. "No matter how populated this place can get, it can seem pretty isolated sometimes."

With accidents like this in mind, rangers at the C&O National Historical Park launched a program last month called Towpath Trailblazers, a volunteer bike patrol that roams up and down the towpath dispensing plastic bandages, tire patches and trail maps.

Armed with two-way radios, first-aid kits and bike repair tools, the volunteers ride in pairs during four-hour shifts along the dusty path, which is sandwiched for 184 miles between the C&O Canal and the Potomac River, from Georgetown to Cumberland, Md.

About 60 volunteers in blue-and-white vests patrol the 30 miles of towpath in Montgomery County and the District. Another 40 traverse the more isolated stretches of the northern and western reaches of the path as it snakes next to the West Virginia border.

"We view the bike patrol volunteers as goodwill ambassadors for the park," said Dirk Helder, an interpretive ranger at the park's Great Falls Tavern Visitor Center. "This is a way to connect the community with the park. We recognize the fact that we need some help."

Although there are about 3 million visitors to the park each year, most of them in Montgomery and the District, only four law enforcement rangers patrol the towpath in the area.

This disparity between the number of visitors and park personnel propelled Grant Reynolds, who lives in Potomac and has biked the path for the last 30 years, to join the bike patrol.

"The National Park Service is so understaffed it makes you want to cry," said Reynolds, 63. "This is a way we can help out."

Reynolds and his wife, Jo, 62, ride from eight to 10 miles each time they go out, mainly handing out maps and reminding visitors to keep their dogs on leashes.

"We've realized we're more approachable if we've stopped. Sometimes we strike up a conversation," said Jo Reynolds, who estimates she and her husband talk with about a dozen visitors each time they patrol the towpath. "There's a fine line between people who want to be left alone to enjoy the solitude and those who want information."

In 1998, there were 41 reported accidents on the 33 miles of the towpath beginning in Georgetown, 250 people who needed minor first aid and 20 search and rescue cases, according to Nancy Brown, volunteer and partnership coordinator for the park.

In the first two weeks of the bike patrol, volunteers assisted 376 visitors.

Brown patterned the C&O program after a similar bike patrol at the Cuyahoga Valley National Recreation Area, outside of Cleveland. Volunteers attend 20 hours of training, which covers use of the two-way radio, cardiopulmonary resuscitation and first aid, bike repair, communication skills and interpretation of the park's plants, animals and history.

"We had a better response than we anticipated when we started [advertising for volunteers]," Brown said. "I think bicycling is really appealing. People are into hiking, the outdoors, exercise. This strikes all of that."

One recent weekday morning, when the air was so thick with humidity that she felt like she was pedaling in a sauna, volunteer Margaret Kilby racked up 38 miles on the towpath. She was slathered with sunscreen and bug repellent, and towpath dust flew up and coated her arms and legs. Nevertheless, she said she was happy to be patrolling.

"I never knew there were bald eagles out there before. We've seen all sorts of birds," said Kilby, 43, of Northwest Washington. "I think we help other people on the canal feel safer. People are always waving to us. They come up to us and say 'I'm glad to see you're here.' "

Park rangers are planning the next training session for bike patrol volunteers in August. For more information, call 301-767-3706.

CAPTION: Jo Reynolds, right, checks the supply of maps and first-aid supplies on the back of Grant Reynolds's bike.

CAPTION: Jo and Grant Reynolds are among about 100 volunteers who patrol different stretches of the C&O Canal towpath. Below, the logo worn by volunteers.

CAPTION: From left, volunteers Jo and Grant Reynolds give information about the canal to hikers Joan and Alan Whelihan. The Reynoldses typically ride from eight to 10 miles on patrol, hand out maps and remind visitors to keep their dogs on leashes.