CREATE TABLE table_name_31 (year_completed INTEGER, dam_type VARCHAR(255), impounded_body_of_water VARCHAR(255))
Which Year completed has a Dam type of concrete gravity, and an Impounded body of water of deep creek reservoir?
    
SQL: SELECT MIN(year_completed) FROM table_name_31 WHERE dam_type = "concrete gravity" AND impounded_body_of_water = "deep creek reservoir"