MEASUREMENTS OF TRANSPARENCY IN THE ROCK RIVER AND ITS TRIBUTARIES IN SOUTHWESTERN MINNESOTA AND NORTHWESTERN IOWA

The Rock River and its tributaries drain a broad plain of Pre-Wisconsin till covered by loess in southwestern Minnesota and northwestern Iowa. Over the past four years, more than seven hundred individual transparency measurements have been made throughout the Rock River watershed.

One particularly distinctive recurrent pattern is a downstream transparency decrease in the tributary called Kanaranzi Creek. The transparency variations documented in Kanaranzi Creek are also observed in other major tributaries including Chanarambie and Champepadan Creeks. The downstream transparency decrease is less distinct in smaller tributaries and in the main stem of the Rock River. However, a similar temporal variation is also documented at several gaging stations along the Rock River where transparency values are inversely proportional to discharge.

These data reflect the expected inverse relationship between transparency and discharge when the suspended load increases with increasing flow rates. Furthermore, perturbations from this pattern in space and time are clearly related to precipitation events and possibly to land use differences within the Rock River watershed.