THE PILOT WAS LANDING ON A GRAVEL BAR IN A RIVER AT HIS HUNTING BASE CAMP. DURING THE APPROACH HE NOTED THAT HE WOULD BE LANDING WITH A LEFT TO RIGHT CROSS WIND. AFTER TOUCHDOWN, HE RAISED THE FLAPS AND APPLIED BRAKES. AS THE BRAKES WERE APPLIED, THE TAIL CAME UP AND THE AIRPLANE NOSED OVER. AFTER EXTRICATING HIMSELF FROM THE AIRPLANE AND WALKING BACK TO THE BASE CAMP, HE NOTICED THAT THE WIND WAS VARIABLE AND HAD SHIFTED TO A TAIL WIND. HE ESTIMATED THE WIND SPEED TO BE 15, GUSTING TO 20 KNOTS.