The Calabar River was once a major source of slaves brought down from the interior to be shipped west in the Atlantic slave trade. Slaving was suppressed by 1860, but the port of Calabar remained important for export of palm oil and other products, until it was eclipsed by Port Harcourt in the 1920s. With improved roads into the interior, Calabar has regained importance as a port and is growing rapidly. The tropical rain forest in the Calabar River basin is rapidly being destroyed, and pollution is decreasing fish and shrimp catches in the estuary. Those that are caught have unsafe levels of contaminants.
The Calabar River drains part of the Oban Hills in the Cross River National Park. The geology of the river basin includes the Pre-Cambrian Oban Massif, Cretaceous sediments of the Calabar flank and the recent Niger Delta sedimentary basin. The basin is about 43 kilometres (27 mi) wide and 62 kilometres (39 mi) long, with an area of 1,514 square kilometres (585 sq mi) At one time it was entirely covered by tropical rainforest.
The region has a rainy season from April until October, during which 80% of the annual rain falls, with peaks in June and September. Annual rainfall averages 1,830 millimetres (72 in). Average temperatures range from 24 °C (75 °F) in August to 30 °C (86 °F) in February. Relative humidity is high, between 80% and 100%. The basin has 223 streams with a total length of 516 kilometres (321 mi). This is a small number given the size of the basin. Drainage is poor, so the basin is subject to flooding, gully erosion and landslides. A 2010 study said that flooding had increased in recent years.
In 1862 the Zoological Society of London received a description of a new crocodile named Crocodilus frontatus that had been taken from the Old Calabar River, with a much broader head than in Crocodilus vulgaris. A new bat called Sphyrocephalus labrosus was also reported.
The river system formed by the Cross River, Calabar, Great Kwa and other tributaries forms extensive flood plains and wetlands that empty into the Cross River estuary. The system has an estimated area of 54,000 square kilometres (21,000 sq mi) As of 2000 about 8,000 tonnes of fish and 20,000 tonnes of shrimp were being caught annually. Shrimp provide a relatively cheap form of protein to the people of Calabar. The fishermen land their catches at Alepan's beach on the Calabar River, and the catch is sold in the surrounding markets.