It's off the GW Memorial Parkway off the bike Trail near Riverside Park. The purpose of the sign is:
Little Hunting Creek E-69 The Washington family land south of here, named Mount Vernon in the 1740s, was part of a grant made in 1677 by the Northern Neck proprietors to Col. Nicholas Spencer and Lt. Col. John Washington, George Washington's great-grandfather. John Washington's son Lawrence Washington, took possession of the eastern half of the grant on Little Hunting Creek. George Washington inherited it in 1761. Across Little Hunting Creek, the Brent family also was granted land in the 17th century. Margaret Brent, secretary to Lord Baltimore, is regarded as the first woman in the British colonies to demand the right to vote.

Little Hunting Creek --- How timely of you to post this sign. On Saturday, April 14, the 24th annual Potomac River watershed clean-up was sponsored by the Alice Ferguson Foundation. At 10 sites along the creek, 139 volunteers picked up 245 bags of trash, 27 tires, numerous bicycles, and 49 shopping carts. Most of the shopping carts were from Walmart and were exhumed from the creek near the bridge at Janna Lee Avenue and behind Sequoyah condominiums. check out the photos at http://www.friendsoflittlehuntingcreek.org

Dave is right. The sign is at the parking lot for Riverside Park near the mouth of Little Hunting Creek. A beautiful spot to pull off the parkway and view the river, especially at sunset.