Mill Stream, a tributary of the St. Croix River, is located in northeren Washington County. The area is characterized by steep hills pock-marked with deep depressions. The stream itself is subdivided into threereaches. The upper reach meanders slowly through and fine sand with bank-full conditions near elevations of adjacent wetland complexes. The middle reach of the stream is the most diverse and altered. It flows through a series of small ponds and flow structures, over bedrock and cobble areas with low sinuosity, and finally through a riffle/pool gravel dominated channel. The lower reach is an entrenched, san dominated channel, significantly altered by a constructed pond and land usa adjacent to the stream. Brook trout are found in all reaches of the stream.

The land surrounding both sampling sites is dominated by roadless, scattered residential and village areas. The stream is rocky-bottomed with plenty of riffles.

The Marine on St. Croix Water Management Organization encompasses 29 square miles and certain portions of the following communites: Marine on St. Croix, May Township, New Scandia Township, and Stillwater Township.

Mill Stream is monitored by the Mill Stream Association.