Despite last-minute personnel changes and a disheartening exodus at intermission time, multi-instrumentalist and composer Sam Rivers and his Rivbea Orchestra did their best to bring the Smithsonian Jazz Celebration at Baird Auditorium Sunday night to an exultant close.

Rivers' avant-garde leanings were apparently a bit much for some listeners, but for those familiar with his work, the real problem lay elsewhere. All told, there were 12 musicians on stage--about twice the number Rivers has recorded with in recent years, yet only a third of the number called for by some of his more ambitious orchestral works. Consequently, the ensemble passages often lacked power and authority. Indeed, some were tentative. And the subtle-yet-striking relationships he's developed with such musicians as Joe Daley--who didn't perform as scheduled--were also missing.

There were compensations, however. Rivers' contrapuntal forces demand strong soloists; he had them in saxophonists Chico Freeman, Bobby Watson, trumpeters Wallace Roney, Jack Walrath, and others. Whenever these musicians carried the weight of the orchestrations, or whenever the ensemble took a familiar route, as it did on the riff-based "Rose," the orchestra seemed just the right size, the extended compositions just the right length.