LONG-TERM WATER TEMPERATURE MONITORING ON THE ST. CROIX NATIONAL SCENIC 
RIVERWAY.

Randy Ferrin and Byron Karns.
St. Croix National Scenic Riverway, National Park Service, P.O. Box 708, 401 
Hamilton St., St. Croix Falls, WI  54024.

The St. Croix National Scenic Riverway (Riverway), one of the original 
components of the National Wild and Scenic River System, hosts an incredible 
diversity of aquatic life. Protecting and enhancing that diversity is an 
important goal of the National Park Service and our cooperating state and 
federal agencies.  The Riverway's biodiversity is a reflection of the good water 
quality and habitat found within the river.  A key abiotic characteristic of the 
Riverway is its water temperature.  The influence of water temperature on 
aquatic life and ecosystem processes is well documented.  With the mounting 
evidence of global warming, and concern about local influences on water 
temperature (such as loss of riparian vegetation, changes in runoff patterns, 
loss of groundwater contribution, and increasing development and impervious 
surfaces in its watershed), Riverway managers felt the need to establish a long-
term water temperature monitoring network.  This information will aid river 
management partners in future decision-making regarding land use planning and 
river management, determine if remedial actions are needed, and provide 
important baseline data for aquatic research. 

To implement this program, a series of HOBO® recording thermometers have been 
placed in the St. Croix and Namekagon Rivers, from the headwaters to Stillwater, 
MN.  A standard protocol is used for the placement of each thermometer. For over 
three years these HOBOs ® have recorded temperature every 15 minutes from May 
through October.  The data is analyzed annually (and graphically highlighted) to 
note changes and trends.  Comparison is made with ambient air temperatures. This 
long-term monitoring activity will be used to help pinpoint possible changes and 
threats to Riverway habitat and sensitive species.

Keywords: water temperature, St. Croix River, HOBO, development, runoff