Once again, beginners and experienced sport anglers are invited to come downtown to join in a celebration of the rebirth of the Potomac River. It promises to be a grand affair, starting with a youth fishing clinic this Saturday at Constitution Lake on the Mall near 18th Street and Constitution Avenue. The clinic will be followed by the weeklong annual TV-7 Fishing Derby, Sunday through June 25. Best of all, everything is free.

Saturday's clinic is tailored for anyone who'd like to know why more than 60 million Americans make fishing their favorite recreational activity. Members of the National Capital Casting Club will demonstrate casting techniques; biologists will teach you about fish species; and expert local fishermen will be on hand to show how to hook a fish. Even equipment will be provided to newcomers. The clinic runs in 11/2-hour segments continuously from 10 to 4.

For the fishing derby, all you need to know is that you can fish the Potomac anywhere between Cabin John Bridge in Montgomery County and Pohick Bay in northern Virginia. That includes all of downtown Washington, along with the Tidal Basin, Washington Channel and those portions of the C&O Canal that fall within the boundaries.

There are two divisions -- for kids up to 12 years of age and adults (anyone over 12) -- and eight categories of competition for prizes that include a sport-fishing trip to Cape Cod, a trip to Orlando, Florida, tackle, trophies and outings to Kings Dominion and Wild World. So get out there and try to catch the biggest largemouth bass, smallmouth bass, catfish, carp, striped bass, crappie, sunfish or perch. After hooking one or more of these fish, take them to Fletcher's Boathouse off Canal Road; the Tidal Basin paddleboat center; the tip of the D.C. Municipal Pier; Columbia Island Yacht Basin in the Pentagon lagoon; or Pohick Bay Regional Park near Lorton.

Prizes and trophies will be awarded June 26 at 3 at the park at Seventh and Water streets SW. Call 572-5688 for information. The entire affair is the brainchild of the Washington Area Waterfront Action Group and is sponsored by WJLA TV-7, Safeway, Schlitz and Outdoor Life Unlimited.