Table schema:
CREATE TABLE table_name_31 (year_completed INTEGER, dam_type VARCHAR, impounded_body_of_water VARCHAR)

Question: 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"