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crater lake (hydrology) A fresh-water lake formed by the accumulation of rain and groundwater in a caldera or crater. A crater lake is a lake that forms in a volcanic crater, caldera, or maar. Incoming precipitation fills the depression to form a deepening lake, until an equilibrium is reached between the rate of water coming in and the rate of water loss due to evaporation, subsurface drainage, and possibly also surface outflow if the lake fills the crater up to the lowest point on its rim. Crater lakes covering active (fumarolic) volcanic vents are often known as volcanic lakes, and the water within them is typically acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes is often exceptional due to the lack of inflowing streams and sediment.