The Groot-Kei (Great Kei River) is a significant river system in South Africa's Eastern Cape Province, formed by the confluence of the Black Kei (Swart-Kei) and White Kei (Wit-Kei) rivers northeast of Cathcart. Originating in the Stormberg Mountains at an elevation of approximately 1,500 meters (4,900 feet), the river flows for 320 kilometers before emptying into the Indian Ocean at the Great Kei Estuary, with the small town of Kei Mouth situated on the western bank. The river historically marked the southwestern boundary of the Transkei region and today forms part of the Eastern Cape's Wild Coast corridor — one of South Africa's most ecologically rich and scenic coastal zones. The estuary and lower reaches are classified as subtropical, characterized by warm waters exceeding 16°C year-round, with minimum winter temperatures ranging from 12–14°C and rainfall distributed throughout the year, creating a warm and humid climate that supports diverse aquatic life.