Theodore Roosevelt Island is getting a bridge, after 200 years of earthen causeways, because silt from the Potomac River is threatening to make the island part of the Virginia mainland.

The National Park Service has been trying to get permission to build the bridge for more than 15 years. Until last year, however, the Park Service had been unable to convince Congress that a $550,000 bridge was needed to the popular but uninhabited island opposite the Kennedy Center.

The small, woodsy island, designated in 1932 as a living memorial to the 26th president, has had earthen causeways connecting it to Virginia since the 1790s when it was the plantation home of John Mason, son of George Mason of Gunston Hall.

The causeways are washed away periodically whenever the Potomac floods, most recently in 1972 during Tropical Storm Agnes. The Park Service spent $15,000 that year repairing the two present causeways, one at the north end of the island which has been used for Park Service maintenance vehicles and one in the center used by the public. The bridge is to replace the center causeway.

The new bridge, scheduled to open June 1, will be concrete with wood railings and will be 12 feet wide, sufficient to carry small Park Service vehicles, according to George Berklacy, spokesman for National Capital Region Parks. The 90-acre island, with its narrow woodland trails, is open only to foot travlers.

The primary reason a new bridge is needed is the heavy river silt, which is rapidly filling a nearly 500-foot-wide channel, once known as Little River, between the island and the Virginia shore. "There would be no island within 15 years if we allowed the siltation to continue," Berklacy said.

The Potomac has been particualrly muddy during the past two decades because of run-off from construction sites upstream. But even the normal river flow in the 18th and 19th centuries caused enough silting to turn the narrow part of the river into shallow, swampy areas perfect as breeding grounds for mosquitoes.

The siltation "has rendered its vicinity unhealthy, in an eminent degree, and will, until its removal, render the island uninhabitable," an Arlington resident said in 1852, two decades after the Mason family left the island and their plantation home because they considered it unhealthy.

The causeway was swept away in a flood later that year but was rebuilt, only to be wrecked again in 1877. Between 1852 and 1877, it was used as a picnic spot, a Confederate recruiting post and finally was occupied by a company of black Union soldiers until the end of the Civil War.

The island subsequently was owned by an athletic club, a boat club and the Washington Gas Light Co. before the Theodore Roosevelt Memorial Association bought it and donated the island to the federal government.

In the early part of this century, the island could be reached only by boat. A cable barge connected the island with Georgetown for a number of years before the Park Service began building causeways in the 1950s. The barges were the main way of crossing the river during the 19th century, before the Long Bridge was built where the 14th Street Bridge now stands.Wagons used the causeway to get onto the island and to the ferry landing.

The new 500-foot bridge is not the first bridge on the island nor the longest. The Theodore Roosevelt Bridge, designed to bring traffic from 1-66 into downtown Washington, was built across the bottom tip of the island in 1959 despite strong objections from enviromentalists contended that the bridge would desecrate the nature sanctuary dedicated to President Roosevelt as well as the George Washington Memorial Parkway and the Mall by the Lincoln Memorial where the bridge ends.