The Army Corps of Engineers and the D.C. government have agreed on a $3.5 million plan for modifying the Little Falls Dam on the Potomac River, where a peculiar hydraulic effect has contributed to numerous drownings and earned the dam the nickname, "the drowning machine."

Rep. Frank R. Wolf (R-Va.) and Rep. Michael D. Barnes (D-Md.) announced the agreement yesterday and said the money will come from funds left over from another project so further congressional appropriations will not be needed.

In the past decade, 22 people have drowned at the dam, which is part of a drinking water intake and spans the river two miles upstream from Chain Bridge.

Water cascades over it and hits a ledge at the bottom, creating a powerful backflow that can trap anyone who enters it.

The Corps of Engineers plans to sink bags filled with grout -- a cement-like material -- to try to slow the rush of water at the dam and lessen the turbulence.

Wolf and Barnes also renewed their efforts yesterday to get the Interior Department to impose mandatory $200 fines for swimming in parts of the river where swimming is prohibited. The maximum fine is $500, but generally a $25 fine is levied.

In a letter to Interior Secretary Donald P. Hodel, Wolf said the disappearance of a 19-year-old Virginian while swimming near Great Falls last Sunday illustrates the need for bigger fines to deter swimming.

The congressmen believe that the higher fine would deter illegal activities in the river. A spokesman for Wolf added that it cost the government $3,000 to rescue five people by helicopter last weekend.