A bog is a wetland type that accumulates acidic peat, a deposit of dead plant material. The term peat bog in common usage is not entirely redundant, although it would be proper to call these sphagnum bogs if the peat is composed mostly of acidophilic moss (peat moss or Sphagnum spp.). Lichens are a principal component of peat in the far north. Moisture is provided entirely by precipitation, and for this reason bog waters are acidic and termed ombrotrophic (or cloud-fed), which accounts for their low plant nutrient status. Excess rainfall outflows, with dissolved tannins from the plant matter giving a distinctive tan colour to bog waters. See also blackwater river.