Why are Senegal and the Gambia two separate countries? What river flows through this area?

The source is only 200 miles (320 kilometers) north of the Atlantic Ocean. The highlands extend to three more countries namely Sierra Leone, Liberia, and Ivory Coast. Other than the Niger River, the highlands are a source of 22 other rivers including Gambia and Senegal.
In 1888, alarmed by French influence in Senegal, Britain seized the river and the land on both sides of it; thus The Gambia became a separate country, the downstream part of the country being a colony and the upstream part a protectorate, and a Gambian legislature was established.