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Environmental Indicators Online

Condition of Rivers and Streams in the St. Croix River Basin in Minnesota

The rivers and streams of the St. Croix River watershed are among the most scenic in Minnesota.  The St. Croix itself has been designated as a National Wild and Scenic River because of its importance as a home for a rich variety of plants and animals as well as a place for fishing, canoeing, and other recreational activities.  Both the St Croix and the Kettle River, one of its major tributaries, are listed as Outstanding Resource Value Waters, a designation given to Minnesota waters deserving of special protection.

The St. Croix today remains one of the state’s cleanest and highly valued rivers.  But a recent study of St. Croix tributaries by a team of MPCA biologists reveals a river system under increasing threat from development particularly in the southern part of the basin where rapid growth of the Twin Cities metropolitan area continues.

This indicator online describes results from this study, which combined biological, chemical and physical indicators to provide a more detailed picture of river and stream quality in the St. Croix basin than has been available in the past.

A 12-page summary bulletin and the full report describing this study are available at the following links: 

• A Comprehensive Assessment of Rivers and Streams in the St. Croix River Basin Using a Random Site-Selection Process
• Fish Biomonitoring Report

For much of its length the St. Croix River runs along the Minnesota-Wisconsin border and its basin encompasses parts of both states (See Figure 1).  Sites for this study, which focused on streams in Minnesota, were selected using a statistically based random site-selection process so that results could be extrapolated to the entire group of streams. 

Figure 1: Map of St. Croix River Basin

Biological assessment methods are not yet fully developed for larger rivers so assessments used in the study were restricted to “wadeable” streams.  In the St. Croix, this represents approximately 70% of the total river miles in the basin.

Biological monitoring

Biological monitoring measures the health of the aquatic community directly, taking into account the effects over time of a variety of stressors.  The effects of these stressors are combined into an index of biological integrity (IBI), which provides a framework for assessing water quality through biology. A typical IBI looks at seven to 12 community attributes such as the number of species, the number of different types of feeding and reproductive groups and the condition of individual organisms.  IBIs are measured on a scale from 0 to 100 with a score of 0 indicating the most severely degraded condition and a score of 100 indicating the highest attainable condition.

MPCA biologists determined IBI scores for fish and macroinvertebrates (mostly insects that spend at least a portion of their lives in water) in the study streams.  Scores ranged from 8 to 97 for fish and from 11 to 92 for macroinvertebrates.  The results indicate that an estimated 29 percent of streams in the basin are impaired for fish and, similarly, 24 percent are impaired for invertebrates.  Combining the two for an overall assessment (a stream is considered impaired if either fish or invertebrates show impairment), 36 percent of streams in the St. Croix River Basin do not support a healthy aquatic community. (Figure 2—Condition of Streams in the St. Croix River Basin).  Figure 3 shows the stream locations that were impaired for fish and macroinvertebrates as well as overall biological impairment.

Figure 2—Condition of Streams in the St. Croix River Basin

Figure 3—Overall Biological Assessment

Habitat assessments

MPCA scientists also used physical monitoring to measure the condition of stream habitat.  Habitat is not often measured or well addressed by existing water programs, but it is just as important to the health of the aquatic community as is clean water.  Habitat varied across the St. Croix basin, with 26 percent of streams in poor condition.  The importance of habitat to the aquatic communities was evident:  58 percent of the streams with poorer habitat were biologically impaired, compared to only 20 percent of streams with better habitat (Figure 4).

Figure 4—Biological Impairment in Relation to Habitat

Water chemistry monitoring

In addition to biological monitoring and habitat assessment, staff also sampled the streams for pH, turbidity and dissolved oxygen and other water chemistry parameters. These results indicated that water quality was quite good.  While 31 percent of streams had single-visit measurements that failed to meet the state water quality standard for at least one chemical parameter, the large majority of those exceedances were on wetland-influenced headwater streams where low dissolved-oxygen and pH levels likely result from natural conditions rather than from human activities.

A wetland-influenced headwater stream (Gillespie Brook) in the St. Croix River Basin

A moderate-size stream (Upper Tamarack River) in the St. Croix River Basin

Water chemistry alone, however, is not necessarily adequate as an indicator of impairment.  Only 37 percent of biologically impaired streams had chemistry measurements that failed to meet standards. 

Effects of land use and ecoregion on biological impairment

Study researchers also found a link between biological impairment and land use in the basin.  IBI scores were 25 points lower, on average, in streams with more-disturbed watersheds where more than 40 percent of the land is under some type of development—agricultural, residential, urban or mining—than in streams with less disturbed watersheds.

The contrast is even more distinct for impairment. An estimated 100 percent of streams with more-disturbed watersheds are impaired, compared to only 11 percent of streams with less-disturbed watersheds (Figure 5).

Figure 5—Biological Impairment in Relation to Land-Use Disturbance

Closely related to land use are ecoregion differences.  Minnesota is divided into seven ecoregions—environmental areas characterized by specific land uses, soil types, topography and natural vegetation.  The vast majority of the St. Croix River Basin lies in two different ecoregions:  the North Central Hardwood Forests (NCHF) in the southern half of the basin and the Northern Lakes and Forests (NLF) in the north.  The agricultural landscape of the NCHF contrasts sharply with the forests of the NLF. 

These differences coupled with the increasing urbanization around the Twin Cities mean that the NCHF southern portion of the basin is much more developed; its average stream has 60 percent of the watershed under development of some kind.  In contrast, in the northern NLF, the figure is only 15 percent.  By the same token, fully 89 percent of the streams in the NCHF southern part of the basin are impaired, compared to only 13 percent in the NLF northern portion. (Figure 6).

Figure 6—Biological Impairment in Relation to Ecoregion

The effects of land use on habitat are almost as pronounced.  Eighty-eight percent of streams with more-disturbed watersheds (more than 40 percent developed) scored in the lower half of the habitat scale.  By contrast only 26 percent of streams with less-disturbed watersheds did so.  The stressors that often accompany development can negatively affect habitat by changing streambed material, by disturbing stream banks, and by reducing a stream’s twists and turns, vegetative cover and variations in other stream features.  Not surprisingly, channel modification has an extreme effect, and results in some of the poorest habitat in the basin. 

Similarly, water chemistry, particularly turbidity and dissolved oxygen, is generally poorer in streams with more disturbed watersheds.

Future considerations

While this study did not assess the St. Croix itself for impairment, similar monitoring conducted in the past suggests that the river main stem is in very good condition.  Likewise, most tributaries, particularly in the northern half of the basin, are also in good condition.  Developmental pressures in the southern part of the basin, however, have had negative effects—physical, chemical and biological—on a sizable portion of the area’s streams.  While these impairments have not yet affected the main stem of the river, the developmental pressures responsible for them are certain to increase as population increases.

Future monitoring will focus on further identifying impaired streams as well as investigating the nature and exact causes of the impairments. This information is needed to address problems in the basin in the most cost-effective way.  Subsequent monitoring will then track water quality changes over time to measure success in resolving those problems. 

For more information on the St. Croix River and its tributaries, see the following web links.

• St. Croix Basin Nutrient Goals
• St. Croix River Basin
• Wisconsin Department of Natural Resources St. Croix River Basin Information