Sometimes the score on the North Central Railroad Trail is Marvin 1, beavers 0. Sometimes, it's Marvin 0, beavers 1.
The only sure thing is the contest, a daily event in which bicyclist meets beavers near the 17-mile marker. The marker is between Bentley Springs and Freeland in upper Baltimore County, on the northern section of the 21-mile trail.
Marvin Yaker says he is not anti-beaver. The 65-year-old retired Baltimore housing official would be willing to live and let dam, if the beavers hadn't dammed Bee Tree Run, causing water to seep onto the trail.
The beavers have become a tourist attraction and the site is one of few places in Maryland where humans and the animals can get close to one another. But trout can't live where beavers dam, and erosion caused by the backed-up stream eventually would mean an expensive resurfacing for the trail.
The challenge for the state Department of Natural Resources is to keep beaver fans, the trout fishing set, bikers and hikers all happy -- and to keep down trail maintenance costs.
Mr. Yaker volunteered to help the rangers. Daily, he removes enough sticks to keep the water level down. At that point, he scores the contest at Marvin 1, beavers 0.
But guess what it will be by morning.
"It's unbelievable," Mr. Yaker said. "They're like hydraulic engineers."
The department estimates traffic at 450,000 bikers and hikers a year on the trail, and many of them stop to admire the dams on both sides and to peer at the beaver lodge on the east.
"The beavers have enhanced our operation. We don't have to pay them for entertainment," said ranger Karl McGovern.
Beavers "represent the wilderness to [people]," said Marilyn L. Mause, a DNR biologist.
But the beavers are also ecology changers. They fell trees and build dams, which changes a forest canopy to an open area and warms and slows the stream so that trout can no longer live or spawn in it.
Their efforts also affect man-made structures. After beavers dammed Bee Tree Run west of the trail last year, Michael J. Browning, northern area assistant park manager, noted: "The trail was damp. Then it got wet. Then it got to be a river."
One-quarter of a mile was flooded, exposing the trail to erosion. Park rangers had to lug in five-gallon buckets of gravel mix. If the erosion continues, the trail would have to be resurfaced.
The staff has few good options. The state is running out of places to relocate beavers. They have no natural predators in Central Maryland. When DNR had a trapper kill beavers that dammed the Gunpowder River in 1993, people objected.
Beaver birth control programs -- which have not been attempted in Maryland -- are in trial stages elsewhere. In 1989, a Colorado citizens group persuaded the city of Denver to try beaver vasectomies and tubal ligations instead of killing or relocating the animals.
Sherri Tippie, president of the citizens group Wildlife 2000, said it was almost impossible to assess the results because of interference. The city parks department destroyed dams and lodges, and trappers illegally killed the animals, she said.
Kathi Green, wildlife biologist with the Colorado Division of Wildlife, said beaver sterilizations seem to work, but then fertile beavers turn up in the area.
The Maryland DNR staff had to do something about the Bee Tree Run beavers. Mr. Browning found himself sending staff members out every morning to create leaks in the dam. The beavers "had town meetings at night" to decide how to repair the damage, said Peyton A. Taylor, manager of the Hereford-area North Central Railroad Trail.
The department's wildlife division suggested beaver baffles -- lengths of plastic pipe with holes bored in it. The baffles, threaded through the dam, let out enough water to keep the level down, but not enough to produce the sound of running water, which alerts the beavers.
The beaver baffles appear to have achieved what Ms. Taylor was hoping for: "They get to keep their dam and we get to keep the trail dry."
Now, about the trout.
Beavers "provide wonderful areas" for waterfowl and warm water fish that need ponds, said Ms. Mause. The dams trap silt and eroding soil. But they're not compatible with trout.
Bee Tree Run has "an excellent population of wild, reproducing brown trout," said Charles Gougeon, DNR area fisheries manager. "By the beavers' presence, you are without a doubt threatening the trout's existence," he added.
Trout need water 68 degrees or colder and they need riffles, places where the stream gradient changes and the water runs clear of sediment and is well-oxygenated, to spawn.
Mr. Gougeon said DNR has documented an increase in the fish population in Bee Tree Run since the department stopped stocking rainbow trout in the stream in 1989, but has also seen water temperatures rise.
Tim Feeser, a member of the board of directors of Patapsco Valley Trout Unlimited, grew up fishing in Bee Tree Run. He said he hasn't yet seen any damage from the beavers' presence, adding, "The key word is 'yet.' "
Wally Vedt, owner of the On The Fly fishing supplies store on Monkton Road, expects to see the trout move upstream and probably drop in population as the water warms. Brown trout can live in warmer, slower water, but the native brook trout, already in low numbers in Bee Tree Run, can't, he said.