Soil-disturbing activity can often stir up iron deposits that find their way to streams. 

Why Total Iron Is Important
Iron is the fourth most abundant element, by weight, in the earth's crust. Natural waters contain variable amounts of iron despite its universal distribution and abundance. Iron in groundwater is normally present in the ferrous or bivalent form [Fe++] which is a soluble state. It is easily oxidized to ferric iron [Fe+++] or insoluble iron upon exposure to air. Iron is a trace element required by both plants and animals. It is a vital oxygen transport mechanism in the blood of all vertebrate and some invertebrate animals.

Environmental Impact
Iron in water may be present in varying quantities depending upon the geological area and other chemical components of the waterway. Ferrous Fe++ and ferric Fe+++ ions are the primary forms of concern in the aquatic environment. Other forms may be in either organic or inorganic wastewater streams. The ferrous form Fe++ can persist in water void of dissolved oxygen and usually originates from groundwater or mines that are pumped or drained. Iron in domestic water supply systems stains laundry and porcelain. It appears to be more of a nuisance than a potential health hazard. Taste thresholds of iron in water are 0.1 mg/l for ferrous iron and 0.2 mg/l ferric iron, giving a bitter or an astringent taste. Water used in industrial processes usually contains less than 0.2 mg/l iron. Black or brown swamp waters may contain iron concentrations of several mg/l in the presence or absence of dissolved oxygen, but this iron form has little effect on aquatic life. The current aquatic life standard is 1.0 mg/l based on toxic effects.

See this link for iron testing field kit information.

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