Change Along the St. Croix Riverway, 1990–2020: Satellite Monitoring

Highway 8 crosses the St. Croix River between St. Croix Falls, Wisconsin, and Taylors Falls, Minnesota; the St. Croix Falls Dam is visible beyond the bridge. Credit: National Park Service / T. Gostomski.

The National Park Service's Great Lakes Network watches for landscape disturbances — forest harvest, blowdowns, fire and development — in and around its parks with LandTrendr, an automated change-detection program that reads satellite imagery.

This second look at the St. Croix National Scenic Riverway analysed 31 years of imagery, from 1990 to 2020, to find and measure landscape change in and around the riverway. The study covered six sub-watersheds:

• the Upper Namekagon River;
• the Upper Tamarack River;
• the Clam River;
• the Sunrise River;
• the North Branch of the Sunrise River;
• the Willow River.

They were chosen to span the range of land cover and ownership found across the St. Croix River watershed. Monitoring covered 347,055 hectares (857,591 acres), of which 31,983 hectares (7,903 acres) lay inside the riverway boundary.

Map 1 (left): the St. Croix National Scenic Riverway (grey) and the six sub-watersheds. Map 2 (right): land cover and land use in the analysis area, from the National Land Cover Database. Credit: National Park Service.

Tree harvest and new building

Forest harvest was the most widespread change, both inside and outside the riverway boundary. It was most prominent in the northern sub-watersheds; two of them, the Upper Tamarack and the Upper Namekagon, contain the largest shares of county-owned land.

• Inside the park boundary, forest harvest affected 10.60% of the land (3,389.29 hectares, or 8,375 acres), with the most activity in 2010 and 2011.
• Outside the boundary, harvest took place in every year of the 31-year period, averaging 940.27 hectares (2,323 acres) a year.

Development was the most common disturbance in the southern half of the study area, mostly in the three watersheds closest to Minneapolis. Those watersheds see less disturbance overall, but because the main agent there is development, the changes could weigh more heavily on how the ecosystem works.

Figure 1. Percentage of land disturbed in each sub-watershed, by disturbance agent and year, 1990–2020; the sub-watersheds run from north (Upper Namekagon, top left) to south (Willow). Credit: National Park Service.

Figure 2. Percentage of land disturbed inside and outside the park, by disturbance agent and year; the vertical scale differs between agents. Credit: National Park Service.

Managing effects that flow downstream

Among the network's nine parks, the St. Croix's pattern of disturbance is unique, because the riverway runs from suburban and agricultural land in the south to extensive forest in the north. Half of the watersheds are dominated by development; the other half see steady forest harvest on public and private land.

Little of the land is federally owned. Monitoring of this kind lets the riverway's managers see what is happening around them, anticipate what could happen to the rivers themselves, and work to soften the worst effects.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network, "Change Along the St. Croix Riverway: 1990–2020." https://www.nps.gov/articles/000/change-along-the-st-croix-riverway-1990-2020.htm
• Full report: Kirschbaum, A. A. 2024. Landsat-based monitoring of landscape dynamics at Saint Croix National Scenic Riverway: 1990–2020. Science Report NPS/SR—2024/155. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/2304342
• Rewritten in hubnx's own words.