question: Which Year completed has a Dam type of concrete gravity, and an Impounded body of water of deep creek reservoir?
answer: SELECT MIN(year_completed) FROM table_name_31 WHERE dam_type = "concrete gravity" AND impounded_body_of_water = "deep creek reservoir"
context: CREATE TABLE table_name_31 (year_completed INTEGER, dam_type VARCHAR, impounded_body_of_water VARCHAR)