What is the River Mile with a RDB lock side and a pool length of 55.4?
CREATE TABLE "downstream_pittsburgh_to_olmsted" (
  "locks_dam" text,
  "location_l" text,
  "river_mile" real,
  "lock_side" text,
  "lock_lift_drop_in_feet" text,
  "pool_length_miles" text
);
SELECT "river_mile" FROM "downstream_pittsburgh_to_olmsted" WHERE "lock_side"='rdb' AND "pool_length_miles"='55.4';