Kerala's western coastal belt is relatively flat compared to the eastern region, and is criss-crossed by a network of interconnected brackish canals, lakes, estuaries, and rivers known as the Kerala Backwaters. Kuttanad, also known as "The Rice Bowl of Kerala", has the lowest altitude in India, and is also one of the few places in world where cultivation takes place below sea level. The country's longest lake Vembanad, dominates the backwaters; it lies between Alappuzha and Kochi and is about in area. Around eight percent of India's waterways are found in Kerala. Kerala's 44 rivers include the Periyar; , Bharathapuzha; , Pamba; , Chaliyar; , Kadalundipuzha; , Chalakudipuzha; , Valapattanam; and the Achankovil River; . The average length of the rivers is . Many of the rivers are small and entirely fed by monsoon rain. As Kerala's rivers are small and lacking in delta, they are more prone to environmental effects. The rivers face problems such as sand mining and pollution. The state experiences several natural hazards like landslides, floods and droughts. The state was also affected by the 2004 Indian Ocean tsunami, and in 2018 received the worst flooding in nearly a century.