LATE RWY CHANGE ON RIIVR STAR TO RWY 25L CONTRIBUTES TO ALT DEV OVER GAATE.

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I WAS THE FO ON A FLT TO LOS ANGELES. I WAS THE PF. WE WERE ON THE RIIVR 1 ARR TO LAX AND WERE INSTRUCTED TO EXPECT THE ILS 25L; GS OUT OF SVC. AFTER XING FUELR INTXN WE BEGAN THE APCH. WE WERE PLANNING ON ILS RWY 24R; AND HAD ALREADY BRIEFED THE APCH TO THAT RWY; SO WHEN WE WERE GIVEN THE CHANGE WE HAD TO BRIEF QUICKLY. I HAD NOT REVIEWED THE APCH AS THOROUGHLY AS I WOULD HAVE LIKED AND WAS NOT FAMILIAR WITH ALL OF THE STEP-DOWN FIXES. AS WE CROSSED FUELR INBOUND; I BEGAN MY DSCNT. I ALSO THOUGHT THAT ATC HAD CLRED US FOR THE VISUAL. ANYWAY; DURING MY DSCNT; I WAS TRYING TO KEEP A 3 DEG GS. AFTER XING FUELR; I CALCULATED THAT I WAS FLYING ABOVE THE 3 DEG GS; SO I INCREASED MY DSCNT. SHORTLY AFTER I REALIZED THAT I WAS GETTING LOW AND STARTED TO SLOW MY DSCNT; BUT BY THE TIME I NOTICED I WAS GETTING LOW I CROSSED GAATE INTXN ABOUT 500-700 FT LOW. AT THE TIME I WASN'T TOO CONCERNED AS I THOUGHT WE WERE CLRED FOR THE VISUAL APCH. THE WHOLE TIME DURING THE APCH WE WERE IN VMC AND COULD SEE THE RWY. THERE WAS NO TFC NEAR US DURING THE APCH. AS WE CONTINUED ON THE APCH; I MANAGED TO STABILIZE THE ACFT AND CROSS EACH FIX AT THE REQUIRED ALT. WE LANDED WITHOUT FURTHER INCIDENT. I WILL NOTE THAT ATC NEVER MENTIONED THE ALTDEV.