What was done
The authors analyzed stream flow records (extending up to the year 2002, with lengths ranging from 53 to 101 years) obtained from 36 gauging stations in five major river basins of Minnesota (USA), deriving histories of seven annual stream flow statistics: "mean annual flow, 7-day low flow in winter, 7-day low flow in summer, peak flow due to snow melt runoff, peak flow due to rainfall, as well as high and extreme flow days (number of days with flow rates greater than the mean plus one or two standard deviations, respectively)."

What was learned
Novotny and Stefan report that significant trends were found in each of the seven stream flow statistics throughout the state, but that in most cases "the trends are not monotonic but periodic," and they determined, as might have been expected, that "the mean annual stream flow changes are well correlated with total annual precipitation changes." Most significantly, perhaps, they found that peak flood flows due to snowmelt runoff "are not changing at a significant rate throughout the state," although 7-day low flows or base flows are "increasing in the Red River of the North, Minnesota River and Mississippi River basins during both the summer and winter," that the "low flows are changing at a significant rate in a significant number of stations and at the highest rates in the past 20 years," and that "this finding matches results of other studies which found low flows increasing in the upper Midwest region including Minnesota (Lins and Slack, 1999; Douglas et al., 2000)."

What it means
"In general," in the words of the two researchers, "an increase in mean annual stream flow in Minnesota would be welcome," as "it could provide more aquatic habitat, better water quality, and more recreational opportunities, among other benefits." Likewise, they say that "water quality and aquatic ecosystems should benefit from increases in low flows in both the summer and winter, since water quality stresses are usually largest during low flow periods." In addition, they say that "other good news is that spring floods (from snowmelt), the largest floods in Minnesota, have not been increasing significantly."

Clearly, even in the fabled "Land of Ten Thousand Lakes," i.e., Minnesota (where our immigrant Norwegian ancestors sank their roots and lived), increasing base flows of rivers and streams are only tending to enhance the environment, in response to - or in spite of (take your pick) - the supposedly unprecedented increases in air temperature and atmospheric CO2 concentration that have been experienced concurrently.

References
Douglas, E.M., Vogel, R.M. and Kroll, C.N. 2000. Trends in floods and low flows in the United States: impact of spatial correlation. Journal of Hydrology 240: 90-105.

Lins, H.F. and Slack, J.R. 1999. Streamflow trends in the United States. Geophysical Research Letters 26: 227-230.