Combined Watershed Health Scores
Minnesota Mean and Minimum Watershed Health Scores
Mean (average) Score
The combined watershed health scores are the average of the five component mean scores. The combined scores closely reflect patterns in land use throughout Minnesota that remove permanent vegetation for agricultural production and/or increase impervious surface. The Minnesota, Lower Mississippi, and Red River basins, which are dominated by agricultural practices, have lower combined scores then the forested landscape of northeastern Minnesota. The scores, which range from 40 to 84, clearly show this pattern even though they represent a combined score from the mean of means.
Minimum (lowest) Score
The minimum index scores range from 0 to 36. This value represents the single lowest index score that each watershed received out of all 18 health index values. Northern Minnesota has slightly higher minimum scores, with a variety of the health indices (5 of the 18) providing that low score. In contrast, southern and western Minnesota have lower minimum scores primarily from only two of the 18 indices: Terrestrial Habitat Quality (Biology) and Aquatic Connectivity (Connectivity).
Calculating Combined Scores
Each watershed received two health rankings for each component:
- MEAN - the mean (average) of the 3-5 index scores within each component
- MINIMUM - the lowest index score found within each component
For an overall statewide score, the five component scores for each watershed were further combined in two ways:
- MEAN OF MEANS - the five MEAN component scores were averaged together to create a watershed mean score.
- ABSOLUTE MINIMUM - the MINIMUM index score of all 18 indices became the watershed minimum score.
Why is the combined mean score important?
Compiling the overall mean scores provides an initial synopsis of statewide trends in watershed health, although it masks extreme values within the individual indices. The overall mean can be used to compare similar watersheds, such as headwater watersheds or upstream versus downstream watersheds within the same major river basin, or watersheds within the same ecological classification. (see maps below)
Click map to enlarge and explore Watershed Health Assessments:
Pattern of combined mean scores:
Mean index scores decline north to south, and east to west; with the lowest mean scores in the Red and Minnesota major river basins. These overall scores closely follow land uses that remove permanent vegetation and/or increase impervious surface.
When displayed with the boundaries from other spatial data, relationships between landscape conditions, river basin position, and watershed health begin to emerge. For example, an upstream to downstream decrease in health scores is evident in the Upper Mississippi River Basin. Further exploration would be needed to determine which health stressors accumulate as the river moves downstream, and which health challenges are expanding outward from the metropolitan areas at the mouth of the basin.
By viewing the combined mean together with the 5 component mean scores, the influence of each component on the overall health score becomes apparent.
Component Minimum Scores
Click to enlarge:
Why is the minimum score important?
The minimum is the lowest of the 18 index scores calculated for each watershed. It may indicate an area in need of focus and effort to improve overall watershed health. It may also help identify the most impacted or limiting aspect of the system.
Click map to enlarge and explore Watershed Health Assessments:
Pattern of minimum scores:
The minimum index scores range from 0 to 36 and are from 4 of the 5 components (none from Hydrology). Northern Minnesota has slightly higher minimums with a variety of indices providing the lowest score. In contrast, southern and western Minnesota have lower minimum scores that are primarily two indices: At-Risk Species (Biology) and Aquatic Connectivity (Connectivity).
When viewing the Ecological Classification boundaries, it becomes evident that the highest minimum scores occur in the Peatlands. The map below shows that the minimum index value is from the "At-Risk Species Richness Index".
Minimum Health Index
This map shows which of the 18 health index values had the lowest score in each major watershed.
Each component also has a minimum score based on the lowest health index value. Comparing minimum index values should be done carefully, without emphasis on the final number. Some indices have a clear "0 to 100" scale of values (e.g. Perennial Cover Index); and some are scaled based on the range of results and best available science (e.g. Climate Vulnerability Index).
As expected, none of the watersheds in MN are in perfect health, as most of the major watersheds across the state received an average or slightly above average score. Due to very comprehensive landscape changes and a growing human population, all of the 5 components of watershed health have been degraded to some degree.
- These results indicate that intensely cultivated watersheds are the most degraded due to the loss of perennial cover, hydrologic storage, terrestrial habitat quality, terrestrial habitat connectivity, and riparian connectivity as well as pollution from non-point sources.
- The northeastern portion of the state, which is primarily forested with lower population densities, has been degraded by point-source pollution from mines, extensive forest harvest and airborne sources of mercury. It is also vulnerable to climate change and hydrologic alterations.
- The Twin Cities metropolitan area and developed corridors toward St. Cloud and Rochester are degraded due to impervious surfaces, intensity of water use and point source pollution. They also have limited connectivity due to the intensity of land use that accompanies urban development.
- The highest health scores were consistently found in extreme north central Minnesota, primarily within the Peatlands Ecological Classification. These watersheds have a high percentage of their landscape in wetland and forested cover with correspondingly low development or agricultural use.
Location matters. The healthier watersheds in Minnesota tend to be those furthest upstream. This trend can be seen in the Red, Rainy, Upper Mississippi and St. Croix Basins. Health problems travel through the system, and their cumulative effect reduces the health of the system downstream. Headwater watersheds that “export” water and are not recipients of sediment and contaminants, tend to have higher health scores then their downstream neighbors.
Headwater areas generally include steeper, more complex terrain so are historically less developed. Settlement occurred first in river confluence areas where commerce was occurring and flatter areas lent themselves to agricultural and human development. Over time, urban areas grow outward from their first historic population centers, often following river valleys and level landscape features.
As index and component level scores are refined, the combined statewide scores will better reflect what is known about ecological system health in Minnesota. Each index includes suggested or planned refinements that may be implemented over time.
The project plan includes re-calculating all index scores on a 5-year basis to begin tracking trends in watershed health. At that time, some enhancements to methods and data are anticipated. To create comparable values over time, some original index values may need to be re-calculated using the new methods.
There are also additional indices already identified that may be created prior to the 5-year scheduled update. These additional values will be incorporated into the combined scores and may create some change to overall results. These additional indices include:
|Biology||Aquatic Habitat Quality|
|Geomorphology||Stream Type/Valley Type|
|Water Quality||Water Clarity|