In normal circumstances, the water level at Chebacco Lake rises with the rain, and gradually drops as it redistributes into the watershed. After rainstorms in 2003, David Lash recalled, the level would rise after the rain but never drop, held back by a beaver dam in an adjoining brook.
''Over the course of the whole summer season, before we intervened, the water rose 18 inches," Lash said. It flooded his neighbors' basements and septic systems before the beavers were trapped and the dam was breached.
While the immediate problems have been addressed, Lash's concerns remain.
''I don't know what the point of saturation is, but right now there are 12 to 15 beaver sites within a half mile of the lake," Lash said. ''Four years ago, we had zero. It can't be possible to manage them all. It will be a constant, ongoing challenge if there is going to be that kind of beaver population in the area."
The residents of Chebacco Lake, shared by Essex and Hamilton, are not alone. Across the region, municipalities and private residents are devoting time and money to the control of beavers. Conservation and health officials help steer individuals through the permit process to breach dams that may flood septic systems or create other public health hazards. State and local highway departments clear beaver debris out of culverts. And on the Essex and Parker rivers, state fisheries biologists have identified beavers as an impediment to the spawning of anadromous fish. Taken together, officials say, controlling the beaver has become a significant effort.
Jeff Chelgren, town administrator for Wenham, said there is a ''constant demand on resources to stay on top of flooding issues," in that town. Beaver dams raise the water level and when rain falls, flooding occurs. ''We have less time to react," he said. ''It crops up everywhere there's a stream."
Though he's ''not necessarily an advocate for trapping," Chelgren said it may be time to revisit the 1996 law banning certain types of traps. Since then, estimates of the number of beaver have tripled, according to Chrissie Henner, a biologist with the state Division of Fisheries and Wildlife. Based on a 2001 study, she estimates there are 65,000 to 70,000 beaver in the state. With the population boom, beaver-human conflicts are handled more routinely, but that doesn't mean they've gone away. Particularly in low-lying areas offering potential wetlands habitat, beavers are still making their presence felt, and testing human tolerance.
Last year, the dam on Alewife Brook in Essex pushed up the water level on Chebacco Lake. In acquiring a permit to breach the dam and trap the beavers in the colony, the property owner, with help from the Chebacco Lake Association, successfully argued that the dam interfered with spawning activity in the lake by preventing fish passage between the lake and the Essex River, which leads to the ocean.
''It's 200 acres of spawning ground, and it's getting clogged," said Michael Armstrong, fisheries biologist for the state Division of Marine Fisheries.
The Essex River isn't the only place where there's concern. The division is in the process of obtaining a special permit allowing volunteers from the Parker River Clean Water Association to maintain fish ladders that provide upstream passage for spawning fish such as alewife, considered an important part of the fisheries food chain. ''On the Parker River, [beavers] keep trying to plug the fish ladders," Armstrong said. ''They hate running water, so they instinctively block them up."
Eric Hutchins, Gulf of Maine habitat restoration coordinator for the National Oceanic and Atmospheric Association, said that beavers clogging man-made structures such as fish ladders, dams, and culverts are common along the Northeast coast. ''They can block them up overnight, no problem," Hutchins said. ''You've got to go back every single day."
Armstrong noted that it is fortunate that volunteers handle much of the maintenance on the Parker River. In other areas, state agencies or municipalities are footing the bills for beaver maintenance.
For instance, a state-owned culvert on County Road (Route 1A) in Ipswich is routinely clogged with beaver debris, which is cleaned out by a state highway maintenance crew or the owner of abutting land. Otherwise, says Warren Jepson, the property owner, water floods onto his property. Jon Carlisle, a spokesman for the state Highway Department, said this is something the department also has done in the past in Manchester-by-the-Sea, Haverhill, and elsewhere.
Ipswich Department of Public Works crews regularly clean out culverts in six locations to prevent flooding of roads, said DPW director Robert Gravino. While the costs are absorbed as part of the department budget, Gravino said that the maintenance hours add up. ''We usually send two people in a pickup truck, and it might take two hours, two or three times a week," he said.
Lane Bourn, Conservation Commission chairman in Rowley, noted that in discussing beaver, wildlife biologists often talk of ''carrying capacity," the number of animals that can live in an area based on resources, and ''cultural carrying capacity," the number that can live in an area without disturbing humans. The latter number is roughly four times less than the former, said Bourn, who sees the growth in beaver population occurring at the same time that home development is pushing into low-lying areas, which is desirable for the beaver as well.
''Houses are being built in places next to prospective beaver habitat," often near streams and floodplains, Bourn said. Officials in that town are keeping an eye on dams on the Mill River, and what Bourn said are six to eight more dams on the Town Brook, through the heart of the downtown. Bourn has organized two meetings on beaver management and permits for Jan. 19 at the Rowley Public Library, one for the general public and another for North Shore officials.
Some landowners use water flow systems that maintain water at a level suitable for both beaver and humans. Because it is believed that beavers respond to the sound of running water by finding the source and damming it up, the flow systems are designed to let water pass through the dam without the sound.
Mike Callahan, who runs Hadley-based Beaver Solutions, said he's installed about 400 flow systems. He has completed several in this region. Callahan also traps, but says the downside is that another colony may return to the same site. That's consistent with the experience of officials from West Newbury and Peabody, who say that their beaver complaints frequently occur in the same locations.