Lower water levels along the United States’ major inland river system affects food and feed costs which can add to the cost of many commodities already magnified by months of drought.

Low water levels in a major shipping corridor between St. Louis and Cairo, Illinois has threatened to upset barge traffic since December. National Geographic reports Mississippi River water levels can drop about three feet from December to March as extra feed from the Missouri River is cut off when that river's navigation season ends, but levels are even lower as rain and ice melt have failed to refill reservoirs.

For every six inches of depth the river loses, barges have to subtract about a hundred tons of capacity. National Geographic reports barges may sink as much as four feet less than usual to avoid bottoming out. The low water levels mean barges carrying corn, soybeans and crude oil are carrying lighter loads, and using more fuel to do so. Midwest farmers along the river are paying a dollar more to ship each bushel of crops than are farmers on the lower Mississippi.

Reuters reports between 55 and 65 percent of all U.S. grain and soybean exports are transported to the Gulf of Mexico by way of the Mississippi River.

The costs add up as farmers are paying more for gas to fill equipment and grains to feed livestock. Those costs end up moving along the chain to consumers.

River issues were further complicated this week when a backup caused by a barge accident and oil spill decreased grain loads travelling along the river.

The closure has not yet made a major impact on grain exports.