Science of Watershed Health



Of the lakes and streams assessed for water quality impairments, what percentage fail to meet water quality standards in each watershed?

Why is this important for water quality?

WQ Assessments Health Scores

WQ assessments health score thumbnail

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The federal Clean Water Act requires each state to adopt water quality standards to protect waters from pollution. These standards define how much of a pollutant can be in the water and still allow it to meet beneficial uses, such as for drinking water, fishing and swimming. A water body is designated “impaired” if it fails to meet one or more water quality standards.

The Minnesota Pollution Control Agency (MPCA) is in charge of assessing the lakes and streams in each of its 81 major watersheds on a 10-year cycle. About half of Minnesota's major watersheds have been assessed as of 2014 (Watershed monitoring map).   

The State of Minnesota designates beneficial uses for each waterbody. Each beneficial use comes with a unique set of water quality thresholds. When the waterbody exceeds one of these thresholds the beneficial use is affected.

Multiple physical, chemical, and biological parameters are measured and monitored to assess surface water quality. The most common parameters causing impairments in Minnesota include turbidity, mercury, total phosphorus, PCBs and other exotic chemicals, fecal coliform, impaired biota, and low dissolved oxygen (DO). 

Minnesota's 2014 Proposed Impaired Waters List has been submitted to the EPA for approval. Detail about the assessment and impairment listing process can be found in the MPCA Guidance Manual for Assessing the Quality of MN Surface Waters.






Input Data


Calculating the index

Interpreting the results


Relationship to other health components

Scientific literature support


Confidence in index

Future enhancements


Input Data 

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Calculating the index

MPCA monitors streams, lakes, and wetlands for water quality impairments. The data set for impaired lakes and streams is from the 2014 Impaired Waters List, which is pending final approval from the EPA. Three designated uses are defined by the MN PCA for streams and rivers: aquatic consumption, aquatic recreation, and aquatic life. If a stream reach was assessed for more than one use category each assessment was counted separately.

An overall major watershed score was calculated by including results for all affected uses (aquatic consumption, aquatic recreation, and aquatic life). The percentage of assessments that fail to meet water quality standards was calculated relative to the total number of assessments for each watershed. Some assessments were found to have insufficient data to classify the waterbody as impaired or unimpaired, these results were not considered in the analysis.

Each of the following metrics compare assessment results within a single affected use category. Each metric is calculated at both the major watershed and catchment scale using the following formula:

100 * ( # of assessments that meet water quality standard / Total # of assessments with conclusive results )


Water Quality Index - water quality assessments, aquatic life

Aquatic Life:

The aquatic life metric is calculated using only streams, lakes, and wetlands that have been assessed for aquatic life impairments. The percent of assessments with aquatic life impairments is calculated relative to the total number of assessments for aquatic life (excluding inconclusive results).


Water Quality Index - water quality assessments, aquatic recreationAquatic Recreation:

The aquatic recreation metric is calculated using only streams, lakes, and wetlands that have been assessed for aquatic recreation. The percent of assessments with aquatic recreation impairments is calculated relative to the total number of assessments for aquatic recreation (excluding inconclusive results).


Water Quality Index - water quality assessment


Aquatic Consumption:

The aquatic consumption metric is calculated using only streams, lakes, and wetlands that have been assessed for aquatic consumption. The percent of assessments with aquatic consumption impairments is calculated relative to the total number of assessments for aquatic consumption (excluding inconclusive results).


Interpretation of results

Watersheds with the fewest overall number of impairments are in the north-central and north-eastern portions of the state. Most of the rest of the watersheds have a high to moderate number of lakes and streams that are impaired. These watersheds are concentrated in the southern and far western portions of the state. The lowest scoring watersheds are ones that have experienced the greatest quantity and intensity of land use conversion, altered surface hydrology, and poor riparian connectivity. Watersheds dominated by high density urban, row-crop agriculture, and surface mines show high percentages of all impairment types. Aquatic consumption impairments show consistently low scores throughout the state, due to the airborne pollution delivering mercury to all surface waters.


Relationship to other health components


Activities that alter hydrology may either increase or decrease the rate at which pollutants move into aquatic systems. For example, the removal of vegetation in riparian or upland areas can increase runoff, which can increase the rate of transport for pollutants to streams or lakes.  Adding physical structures to hold water and sediment may decrease runoff in some conditions but release pollutants if structure fails.


Water quality impairments may reflect the presence of structures in streams, rivers and lakes. Streams that are dammed or have culverts that were not appropriately designed create artificial impoundments that collect fine sediment, which can lead to high turbidity. Many impoundments are relatively shallow so pollutants, such as phosphorus, accumulate and are resuspended from the bottom into the water column by wind and waves, which may result in an impairment.  

Lack of riparian connectivity limits the interaction between a water body and its floodplain.  During high water events, the sediment, water, energy and contaminants will be directed downstream rather than being distributed and deposited into the floodplain. 


The geomorphology of the watershed landscape can influence the presence of water quality impairments. For example, if potential pollutants such as sediment, phosphorus or bacteria are present, they will be more readily delivered to water bodies in areas with higher slopes or fine soils. Additionally, landscapes with karst features are at higher risk of delivering pollutants to groundwater.

There is also a direct relationship between altered stream geomorphology and water quality impairments.  When a stream has been altered, its meander pattern no longer fits the slope and size of its valley.  This mis-match leads to headcutting, streambank erosion, and general stream instability that usually increases stream turbidity. 


Many water quality impairments affect aquatic organisms. For example, many heavy metals can be lethal to fish, aquatic invertebrates, or mussels at very low concentrations. High sediment loads may be lethal to aquatic organisms, but more often sediment results in chronic effects, such as reduced growth rates in fish.  High phosphorus levels can accelerate the growth of algae and other plants, depleting oxygen levels and stressing aquatic life. 


Scientific literature support

Most of the listings under toxics are fish consumption advisories related to mercury, Polychlorinated Biphenyls (PCBs), or Perfluorooctane Sulfonate (PFOS). All three of these toxic pollutants are primarily airborne and most often not directly related to anthropogenic changes on the landscape. These airborne pollutants usually do not result in death of aquatic organisms, but are taken up and concentrated in the tissues of fish, which are then eaten by humans.

Eutrophication, characterized by algal blooms, is related to nutrient pollution (usually nitrogen or phosphorus), which are applied as fertilizer to agricultural fields or lawns. Nutrients typically do not cause direct mortality of aquatic organisms, but often cause excessive plant growth. Actively growing plants may remove dissolved oxygen (DO) from the water. Decaying plant material may also remove DO from the water. There are 100 listings on the 2010 303(d) list for low DO. In either case the reduction of DO can cause mortality in aquatic organisms when the concentration falls below 5 mg/L (Davis 1975). Low DO may not cause immediate mortality but can reduce metabolic function of fish (Jobling 1994: 233-235).

Turbidity is usually related to fine sediment that enters waterbodies from agricultural practices, road or home construction, timber harvesting, or other anthropogenic activities that alter the landscape by removing vegetation and exposing soil to erosion. Fine sediment can reduce a number of physiological functions of aquatic organisms, such as growth rate, or may lead to death at very high concentrations (Newcombe and Jensen 1996).

Bacteria enter streams from a number of sources, but most often from cattle grazing near a waterbody, manure lagoons, or leaky septic systems (Baxter-Potter and Gilliland 1988, Krysel et al. 2003, Sherer et al 1988, Tiedemann et al. 1987, 1988). Bacteria usually do not result in mortality of aquatic organisms, but can affect human health.


Confidence in index

The MPCA is the state agency charged with the collection of water quality information in Minnesota. The information is collected following standard published procedures by trained biologists and environmental chemists. Information on impaired waters is complied into the section 303(d) list, which is publically available.


Future enhancements

The distribution of surface water assessments is closely tied to the MPCA watershed monitoring approach (watershed monitoring map).  Each major watershed in the state receives intensive monitoring within the ten year cycle. Periodically, as additional data becomes available, the index scores will be updated to reflect the increased data coverage. As consistent data sampling is repeated over time, indices will be updates so that incrimental change can be observed over that time period.