 The region's tall grasses and extensive marshes make it favourable for birds, reptiles, and large mammals.Chad's major rivers—the Chari, Logone and their tributaries—flow through the southern savannas from the southeast into Lake Chad.Each year a tropical weather system known as the intertropical front crosses Chad from south to north, bringing a wet season that lasts from May to October in the south, and from June to September in the Sahel.