If last year's TV-7 Potomac River Fishing Derby was any indication, you'd better stake out a spot for this popular annual outing. The contest, co-sponsored by government, business and civic groups, starts this Friday and continues until September 5. It's free and open to all.

Eligible fish must come from the Potomac anywhere between the Beltway bridges, the C&O Canal, the Tidal Basin or Washington Channel. Bass, catfish, carp, perch, rockfish, sunfish and crappies can be entered for youth and adult prizes, including a trip to Orlando, a 71/2-hp outboard motor, a mini bass-boat, tackle boxes, rods and reels, a trolling motor, a depthfinder or special awards for tagged fish in the Tidal Basin.

But your catches have to be registered: Visit Fletcher's boathouse off Canal Road or the paddleboat rental center at the Tidal Basin; boaters can register their catch at the Harbormaster's Pier in Washington Channel. Check-in stations will be open from noon to 5 daily. MARYLAND RIVERS AND LAKES WSSC RESERVOIRS -- A 61/2-pound smallmouth on a live crayfish bait? It happened at Triadelphia Reservoir last week in an area of the lake that has tremendous bottom structure. Rocky Gorge Lake has given up some hefty largemouth bass on grubs or plastic worms. Just be there early and catch the fish returning to deep dropoffs after feeding in the shallows. POTOMAC RIVER -- Just in time for the Fishing Derby, bass are taking chocolate-color plastic worms during the wee hours at Washington Channel and in the rocks around Fletcher's. The weather is a major factor. Should thundershowers arrive from the north and west, Montgomery County and downstream portions may become discolored. Don't let that stop you, however. Feisty catfish are more than happy to inhale liver or worm baits from above Hancock (Washington County) all the way down into tidal Charles County. Smallmouth bass waders have scored around Knoxville during late evenings. SUSQUEHANNA RIVER -- The Flats have seen rockfish up to eight and nine pounds taking surface plugs around sunrise. Bass hunters can score inside the river on spinnerbaits, plastic worms and smoke grubs. NANTICOKE RIVER -- Early mornings are fine for bass inside Broad Creek, upstream from the Sharptown (Route 313) area. Surface plugs and buzz baits will draw hits. Later in the day, switch to plastic worms. SOUTHERN MARYLAND LAKES -- My almost- teenage nephew, Lothar, says, "I'm tired of catching the bass down there. I'm ready for the big stuff now." What he refers to, of course, are the thousands of 10- to 12-inch largemouths that go gaga over plastic worms at St. Mary's Lake (off Route 5, south of Leonardtown). Gilbert Run Park lake (Route 6, east of LaPlata) is plugging along with sunfish for the little ones and an occasional whopper channel cat for dad. CHESAPEAKE BAY On-and-off success on sea trout and bluefish may sound like a cop-out to those who need more to go on, but that's the way it is over major portions of the Bay from buoys 50 north to 54. Night anglers still fare better than daytimers. Some anchor along the channel ledges from Sharps Island down to Hooper Island Light and beyond with peeler crab pieces dangling on bottom rigs, and before day arrives something will inhale it. During the day, however, a bright spot has been the Stone Rock area near Sharps Island, where the fleet congregates hoping to find a sea trout, bluefish or even a black drum. Weekends can be busy here. On a sad note, we heard from a reader in Riverdale who complained of a trip to the Point Lookout area aboard a headboat. He said the boat was dirty, the meager amount of bait handed out was too expensive, the heads were filthy and so on. Needless to say, his Elks Club won't be back. What a shame such things ocur. Elsewhere, white perch and snapper bluefish are inside Eastern Bay, as well as in the Kent Narrows and on the eastern side of the Route 50 Bay Bridges. A few large rockfish hang around the mid-channel pilings. VIRGINIA RIVERS AND LAKES LAKE ANNA -- You can find the bass if you stick with it long enough. Plastic worms and jigs attract quite a few up to six pounds along landpoint dropoffs or mid-lake underwater structure. Big catfish and scattered stripers are taken also. OCCOQUAN RESERVOIR -- Catfish are the main fare for bottom bait dunkers. Crappie, although mostly small, are willing. RAPPAHANNOCK RIVER -- Thunderstorms again may muddy the waters, but if not, the smallmouths will take spinners, live bait or surface buzzbaits above I-95 bridge. Largemouth bass activity below Fredericksburg has been fair, with the side creeks around Leedstown producing bass early in the day during tidal changes. In saltwater at the river mouth, some flounder, spot and small bluefish are taken. The Moratico sector offers sea trout. BURKE LAKE -- Slow for bass, but sunfish and catfish are willing. Just be sure to get there early and fish far away from the busy boat-center area. OCEAN AND INLETS MARYLAND -- Offshore headboaters find seabass and tautog in fair numbers, while Assateague Island surf anglers are starting to locate a few sea trout and snapper bluefish in addition to kingfish. The bluewater marlin fleet is doing well on whites and scattered wahoo over the Baltimore and Washington canyons. Bluefish trolling at the Bass Grounds and Jackspot has picked up, but the small- boat flounder drifters have been working harder for their fish this week than last. VIRGINIA -- Too late to be included in last Friday's report, the heaviest fish to be pulled from Virginia waters thus far this season had to be the 729-pound tiger shark boated by Don Lips of Virginia Beach. Lips fished with none other than Gary Seay, the Virginia-shark recordholder, some 30 miles east of Rudee Inlet. Meanwhile, amberjack up to 80 pounds have been delighting Old Dominion offshore trollers around the Chesapeake Light Tower, while the 26-mile Hill has been good to Wachapreague charter skippers looking for tuna. Billfishing is holding its own and the backwaters of Chincoteague, Wachapreague and Oyster are good for mostly small flounder. The Bay Bridge-Tunnel near Cape Charles shows a few sea trout and flounder, and the waters close to land always hold spot in that region.