Color This is a measure of the natural absorbance or color of a water sample. Natural color is usually the result of high concentration of relatively large molecules of dissolved organic matter usually referred to as humic material. Our rivers have high concentrations of these compounds and thus are highly colored. Color is measured relative to a platinum-cobalt standard and the reporting units are “PtCo units”. Typical river water ranges from 180 to 250 PtCo units. Values below 180 are observed immediately after large rain events which dilute the river water. Values greater than 250 are observed following floods or high enough water so as to submerge adjacent swamps. This water leaches organic matter out of the swamps and the water eventually drains back into the river.