Effluent from the homes and eateries along the Northern Cross-Island Highway, and organic phosphates and nitrates from fertilizers used in the mountain orchards, filter through into the rivers, and with an excess of nutrients, the clear water has turned cloudy and poisonous algae have gradually proliferated.
北橫沿途的家庭與餐廳汙水、山坡上果園灑下的肥料等有機磷、氮物質逕投河裡，清水過於營養，成為濁水，毒藻伺機繁衍。