A cheer went up Saturday at 11:27 a.m. at Blob's Park in Jessup, Md., as a truck approached the bandstand -- it bore a piano, nearly an hour late in delivery. Several hundred fans at the Potomac River Jazz Club's 15th Annual Jazz Picnic had already settled into lawn chairs and a few had spread blankets beneath the welcome sun. Others were arranging vast spreads of food on tables in the nearly cavernous pavilion as they anticipated the initial band of the 13 scheduled to play a 40-minute set each.

The 10-member Capital City Jazz Band, a hard-driving unit with a rock-solid beat and the gospel flavor of Estelle Minor's piano, kicked off with Louis Armstrong's "Gut Bucket Blues." Bob Walters' clarion cornet led the pack on the upbeat tunes and delightfully salacious double-entendre lyrics of Bessie Smith and others were offered by Minor's daughters, Lorie and Maria, and the pianist herself.

Brief intermissions afforded the opportunity to pick through a flea market, where vendors hawked PRJC T-shirts, posters, buttons and mugs, new and used records, including rare LPs and antique 78 RPMs, tempting pastries and secondhand instruments. One prospective buyer blew tentative choruses on a contemplated purchase, a vintage though burnished trumpet, to the rapt attention of a gawking 3-year-old boy.

"In a couple of months I'll be an octogenarian," enthused longtime PRJC member John Sears, taking a breather from the dance floor aside the bandstand to explain the source of his energy, good health and high spirits. "It's the Potomac River Jazz Club, its members, its music, the dancing and the friendship that keep me in this area and I have no desire to go anywhere else. My doctors are losing money on me, much to their chagrin."

As the afternoon progressed and attendance topped 1,500, the sun grew warm and one band after another took its place under the yellow and white striped canopy. Baltimore's Pier 5 Jazz Band proved to be a stomping unit with tight ensembles, a crisp attack and soaring solo action. Pianist Ed Fischel's The Band From Tin Pan Alley swung gently but unerringly on a program of Jelly Roll Morton tunes that included "The Pearls" and "Winin' Boy." A first prize for eclecticism should go to another Baltimore combo, The Peabody Ragtime Ensemble, which presented a polished and lively program that covered the spectrum from march to rag to Dixieland to Fats Waller and blew up a hurricane with the Benny Goodman/Gene Krupa war horse "Sing Sing Sing."