16 May 2011

MODIS Reveals Major Sources of Sediment

Posted by John Freeland

Annotated MODIS true color image of Lake Erie, 9 May 2011 (click twice for larger views)

It’s mud season in the Midwest and the MODerate Imaging Spectroradiometer
250m true color band beautifully displays the tell-tale sediment plumes entering western Lake Erie (left). Updates of Great Lakes MODIS imagery are available here

MODIS is a multi-band imaging instrument mounted on two Earth-orbiting satellites, the Terra, and the Aqua, both part of the NASA-led international Earth Observing System. Between the two of them, the entire Earth is imaged every one or two days. Originally intended to observe aerosols, MODIS does a good job identifying where “earth” flows into water.

Two big rivers entering western Lake Erie are the Maumee and the Sandusky, both of which drain clayey soils that support row crops, especially corn. Their brown sediment plumes carrying heavy loads of spring runoff and sediment are shown entering the the southwest corner of the lake. Lake Erie is shallow and wind-swept. Some of the lake turbidity may be due to re-suspension of bottom sediments.

In Canada, the Sydenham and Thames Rivers flow through major agricultural lands and dump sediment into Lake St. Clair, upstream of Lake Erie.

Of major concern from a water quality standpoint are the nutrients and agricultural chemicals adsorbed onto the clay particles carried to the lake and suspended in the water column. Phosphorous overload is responsible for nutrient enrichment (eutrophication), nuisance algal blooms, and a persistent oxygen-deprived dead zone in Lake Erie.

MODIS true color image of Lake Huron 9 May 2011 (click twice for larger views)

The MODIS image of Lake Huron (right) from the same day, however, shows a much different scenario. Aside from a brown sediment plume entering Saginaw Bay at the mouth of the Saginaw River, the lake is almost completely clear. The differences between Lake Erie and Lake Huron watersheds are many but soil composition, forest cover, degree of urbanization and agricultural land use are major contrasting factors.