B777 FLT CREW IS ASSIGNED RIIVR1 ARRIVAL TO LAX WITHOUT RWY. FLC ASSUMES RWY 24R INCORRECTLY AND IS VECTORED FOR VISUAL TO RWY 25L. DESCENT IS STOPPED BY SCT AT 9000 FT.

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FOR THE ARR INTO LAX WE BRIEFED AND SETUP FOR THE RIIVR ONE ARR/PALAC TRANSITION TO RWY 24R. THE ATIS INDICATED LNDGS WERE BEING CONDUCTED TO RWY 24R AND RWY 25L. WE WERE CLRED TO DESCEND VIA THE RIIVR ONE ARR; BUT WERE NOT GIVEN A RWY ASSIGNMENT. AS WE PASSED HABSO; WE WERE STILL AT 14000 FT; AND WERE CONCERNED THAT WE COULD NOT BE ABLE TO COMPLY WITH THE PUBLISHED STEP-DOWNS. ONCE WE RECEIVED OUR CLRNC TO DESCEND; WE HAD TO RUSH TO COMPLY WITH THE ARR PROC. THIS LATE CLRNC FORCED US TO EXPEDITE OUR DESCENT; AND AS WE DESCENDED THROUGH 9000 FT; WE RECEIVED AN UNEXPECTED DIRECTIVE FROM APCH CTL TO LEVEL OFF AT 9000 FT. THOUGH WE HAD ALREADY DESCENDED BELOW 9000 FT IN OUR ATTEMPT TO REACH 7000 FT AT PALAC; WE COMPLIED AND CLBED BACK TO 9000 FT. WE QUERIED THE CTLR ABOUT OUR RWY ASSIGNMENT; AND HE RESPONDED WE WERE CLRED RWY 25L. THIS WAS A CHANGE IN OUR ORIGINAL CLRNC AS WE UNDERSTOOD; AND WE WERE FORCED TO RELOAD THE WAYPOINTS FOR THE NEW RWY. WHEN WE IDENTIFIED THE FIELD VISUALLY; WE WERE GIVEN A VISUAL TO RWY 25L. WE THEN SWITCHED TO TWR AND MADE AN UNEVENTFUL LNDG. LATER WE CONTACTED SOCAL TRACON. THE OPERATION OFFICER CONFIRMED THERE WAS NO VIOLATION OR CONFLICT. TO OUR DISMAY; THEY ADVISED US THAT RWY 25L IS ALWAYS THE DEFAULT RWY; UNLESS ASSIGNED ANOTHER RWY.