Okker dannes, når der opstrøms i vandløbssystemet er sket en grundvandssænkning efter dræning eller vandindvinding. Hvis der er pyrit (svovlkis) i jorden, vil det blive udvasket til bækken, og når pyrit iltes, sker der en kemisk reaktion, som i sidste ende resulterer i, at fri jern udfældes som okker i bækken. Det er jernet, der giver den røde farve.

Okker er et miljøproblem i mange vandløb i det vestlige og sydlige Jylland. Udfældet okker og andre jernforbindelser er en trussel for dyre- og planteliv, og ved meget kraftige koncentrationer dør fisk og andet dyreliv.

Okker forefindes især i den vestlige del af Danmark.

Ocher is formed when there has been a lowering of the groundwater upstream in the stream system after drainage or water extraction. If there is pyrite (sulfur pyrite) in the soil, it will be washed into the stream, and when pyrite is oxidized, a chemical reaction occurs which ultimately results in free iron being precipitated as ocher in the stream. It is the iron that gives the red color.

Ocher is an environmental problem in many streams in western and southern Jutland. Precipitated ocher and other iron compounds are a threat to animal and plant life, and at very high concentrations fish and other wildlife die.

Ocher is found especially in the western part of Denmark.