Which Year completed has a Dam type of concrete gravity, and an Impounded body of water of deep creek reservoir?
CREATE TABLE "major_dams_and_reservoirs" (
  "dam_constructed" text,
  "year_completed" real,
  "impounded_body_of_water" text,
  "reservoir_capacity" text,
  "dam_wall_height" text,
  "dam_type" text
);
SELECT MIN("year_completed") FROM "major_dams_and_reservoirs" WHERE "dam_type"='concrete gravity' AND "impounded_body_of_water"='deep creek reservoir';