lower minnesota river model

By Cathy Larson, Metropolitan Council Environmental Services

For many years, the Metropolitan Council and other groups have been monitoring the Lower Minnesota River, its tributaries, and point-source dischargers like wastewater plants. Last October, the Council modified its monitoring programs in order to collect data needed to build and test an advanced, computer-based water-quality model of the lower 40 miles. The new monitoring program will continue for three years, through 2006, as an integral part of the Lower Minnesota River Model. This spring, the river went from very low flows to flood conditions in a matter of weeks, providing a variety of flows for testing the model. This summer, when river flows are low, we plan more intensive monitoring, as this is the time we might expect severe algal blooms and low oxygen levels.

The river model, which will cost $1 million, is being funded by the Metropolitan Council, U.S. Geological Survey (USGS), Minnesota Pollution Control Agency (MPCA), and Lower Minnesota River Watershed District.

What exactly is a river model?

What, more tangibly, is this water-quality model, and how can we envision it? In truth, it’s a tough thing to explain, but basically the model is, according to an academic source, “a set of equations that attempt[s] to describe all the relevant processes, or mechanisms, in the water body.” For example, if we want to know how algal growth would affect oxygen levels, we need a model that describes the relationship of algae to nutrients, light, temperature, flow, and other growth factors. We've decided to use the CE-QUAL-W2 computer model, which is supported by the U.S. Army Corps of Engineers, has been successfully applied to over 500 aquatic systems, and is well accepted by the scientific and regulatory communities.

Once built, the model will serve as a way to test different water-management strategies. For example, the Minnesota Pollution Control Agency plans to use the model to conduct a wasteload allocation study and establish discharge-permit limitations that will protect water-quality standards. The Metropolitan Council will use the model to plan wastewater facilities and set watershed goals.

Low river flows last summer

Low river flows occurred after the basin entered an extended dry period in mid-July 2003. Flows fell to less than 1500 cubic feet per second (cfs) in late August and September (normal flows are 3300 cfs in August, 2200 cfs in September). While we didn’t have the enhanced monitoring program in place, we did conduct routine monitoring and some special studies. Last summer provided a good test for low flows and yielded some interesting information.

For example, we learned that the problem of low oxygen in the Lower Minnesota River still exists despite upgrades in wastewater treatment and improvements in watershed management.

Mid-August last year, field crews from both the U.S. Geological Survey and Metropolitan Council recorded low dissolved oxygen (DO) levels, of 4–5 mg/L, from the mouth of the river to at least mile 10 upstream. While performance at the Blue Lake and Seneca Wastewater Treatment plants, miles 20.5 and 6.7 up river respectively, was much better than required, the Minnesota River at Jordan had excessive levels of algae and organic matter, which contribute to high oxygen demand.

In short, the Minnesota River showed signs of stress last summer, and conditions would have been worse if river flows had been lower or if Blue Lake and Seneca had been performing at their allowable limits.

Sediment bed & seepage studies

In September 2003, the USGS conducted an assessment of the sediment bed in the lower 26 miles using a seismic profiler. This relatively new technology allows a quick scan of a large section of the river, with which, this winter, the Metropolitan Council compiled a fine-scale sediment-bed map. In addition to the USGS scan, the MPCA conducted a visual survey of the sediment bed in select locations and found a thin layer of silt covering the sediment at most sites. Both assessments show that the sediment bed is dominated by sand-silt and sand-gravel mixtures at low river flows. Silt can indicate the presence of fine organic materials whose decomposition may decrease oxygen available to higher aquatic life forms.

Last summer, the USGS also conducted studies of ground-water seepage and river-mixing characteristics.

The seepage study has helped us understand that direct ground-water discharges to the river — springs and seeps in the banks and bed — do not substantially affect river flow or loads. Therefore, we will not pursue additional field studies to quantify ground-water flows and loads.

The mixing study should demonstrate if and when we need to add vertical and lateral dimensions to our usual single-point, mid-channel measurements. In work to date, we have measured vertical and lateral differences in turbidity, oxygen, temperature, pH, and conductivity at five locations. We will continue the study, and complete it, this summer.

Stream-flow gaging station at Fort Snelling

As reported in another article in this issue of the News, the District and the USGS are partnering on a stream-flow gaging station at Fort Snelling, an important part of the new river model.

Discharge will be added to the Web site later this summer as relationships to stage and velocity are developed. The station will provide more accurate information on flows and pollutant loads leaving the Minnesota River and entering the Mississippi River. By difference, we’ll be able to estimate pollutant loads entering the Minnesota River between the Jordan and Fort Snelling monitoring stations. In addition, the Fort Snelling station will help us understand the dynamics and effects of backwashing from the Mississippi River. We’ve known that backwashing occurs but have not understood the timing or magnitude.