Maryland has about 300 earthen dams monitored by state inspectors. Like the 65-foot-high Need-ville Dam in Rockville, where torrential rain forced the evacuation of local residents this week - some of them are pretty large.
They include the 99-foot-high Little Seneca Lake Dam in Montgomery County and 118-foot-high Druid Hill Lake Dam, according to state and federal Web sites.
But an earthen dam can be just as safe as a concrete structure, experts say.
"If you've got a good site with a good foundation, earthen dams are a good way to go," said Larry Roth, a civil engineer who has designed dams and is deputy executive director of the American Society of Civil Engineers.
Officials estimate that 80 percent of Maryland's 400 dams are earthen. They're built with either a mix of a homogeneous material, such as silt and clay; or with a thick sheet of clay supported on each side by a sloping wall of rock or another granular material, experts say.
"Either way, they act as gravity structures, with the weight of the material holding the water back," Roth said.
The danger comes when water tops the embankment, which can erode the face of the dam as it races down the normally dry side.
Roth said one problem across the country is that many dams are privately owned and owners sometimes say they don't have enough money to make repairs.
Development around dams also has put more people at risk, he said. A recent report card issued by the engineering society gave the nation's dams a "D" because owners are not paying attention to dam maintenance.
"It's a very difficult situation," Roth said.
But inspectors in Maryland say they've have had little trouble persuading dam owners to make repairs when necessary, said Ken Pensyl, program administrator of the sediment, storm water and dam safety program at the state Department of the Environment.
Most of the larger dams are owned by governments or public agencies, he said.
"Everybody realizes that you're dealing with issues that mean a potential loss of life, and there's usually agreement that the work has to be done," Pensyl said.
Larger dams generally are classified into one of three risk categories: high hazard, significant hazard and low hazard. A dam's classification depends on its size and the potential deaths that would occur if it failed.
In Maryland, all high-hazard dams - including the dam in Rockville - are inspected at least once a year, Pensyl said. The Lake Needwood Dam last went through a complete inspection Oct. 17, 2005, and a supplemental inspection in March, Pensyl said.
The state has four inspectors and is planning to hire a fifth. They are responsible for inspecting about 358 dams for any signs of potential problems, he said.
An inspection can take anywhere from a few hours to a few days, he said.