Understanding our streams and rivers: Streamflows
As we saw in the national news again this spring in the Fargo/Moorhead area, rivers flood. They always have and they always will. Even in our own backyard watershed, the Pomme de Terre River crested at the end of March with the sixth highest recorded flow since the USGS established a stream gauge in Appleton in 1931. Looking at the river now, after this dry summer, it's hard to imagine the amount of water the river was carrying.
Of all factors affecting streamflow, the most important is climate. The amount of precipitation is responsible for the volume of flow through a stream over time. The seasonal timing of snow, snowmelt, major rainfall, and dry periods determines the seasonal pattern of streamflow. In the upper Midwest, snowmelt and spring rains correspond with what are usually the highest streamflow of the season; this is often referred to as the "spring flood".
Floods are categorized by their reoccurrence intervals. You'll hear terms like "10-year flood" "100-year flood" "500-year flood" when describing flood events in a river. For example, during the flood of 1997 the Pomme de Terre River crested with a flow of 8,890 cubic feet per second, this was the highest flow on record since 1931. This was an approximately 500-year flood event for this river. In July of 1867, a freak summer storm dumped 30 inches of rain on the area, supposedly, the Pomme de Terre and Chippewa Rivers were so flooded, they actually merged together. It would have been interesting to have a flow gauge operating back then to see what kind of flood event that would correspond to.
A hydrograph is a picture that depicts the seasonal variability of streamflow. On average, most rivers in this region are at their lowest flow in the winter. The flow picks up during the snowmelt period at the end of March, and the rivers usually crest in early April. Then the flow gradually declines through April and May. Usually one or two good storms in June or early July will cause another spike in river flow like we saw in June of 2008. The flow will then decrease throughout the summer back down to what is called "base flow" conditions and stay in that state throughout the fall and winter.
An important facet of natural, stable streams is that they consist of two parts, the channel and the floodplain. Stream channels carry floods that occur two out of every three years, on average. These smaller, more frequent floods carry most of the sediment over time. Larger floods that spill out of the banks and are carried on the floodplain, carry large amounts of sediment but occur less frequently. It's important that during these high flow events, the river is able to access its floodplain. This allows the large flow of water to spread out, dissipate its energy, and allow the sediment it's carrying to settle out. If a river channel is disconnected from its floodplain, either by down cutting, ditching, or levy building, all the energy of the water is contained within the channel. The velocity will increase leading to greater erosion and damage.
The natural cycle of high and low flows is necessary to maintain a natural functioning ecosystem. Floods are necessary to form floodplains, distribute rich alluvial soils, clear organic debris, sort rocks into alternating patterns of bars and riffles, and recharge riparian wetlands and aquifers. During low flow periods, many forms of life can be stressed in the short term. But they also allow seeds to sprout in normally wet areas, helping to reinvigorate floodplain vegetation.
If one looks at the flow data from the Pomme de Terre River, you'll see the average yearly flows are increasing. Not only are the average flows increasing, the yearly peak flows are also increasing. Since 1931, there have been six years where the flows exceeded 3,000 cubic feet per second, three of those were in the 66 years prior to 1997, and three have been in the dozen years since 1997.
I mentioned above, that the amount of precipitation is the most important factor in determining stream flow. But if one looks at the precipitation data, there has been no significant change in the amount of precipitation the area has been receiving. Why is the streamflow increasing when the amount of precipitation has stayed the same?
Human land use has greatly affected the flow of water into rivers and streams. In the Minnesota River Basin, flows have been modified greatly by human activities over the past 100 years. Over 80 percent of the land contained in the basin is under agricultural production. To support agricultural activities, the basin has been extensively ditched, channelized and drained by a vast network of drainage ditches and subsurface tiles. All of this increases the speed at which water runs off the land into nearby streams and lakes and eliminates the storage capacity of wetlands. By speeding the flow of water into rivers and streams, these land use changes alter the hydrology of a stream, causing streamflows to rise faster and peak at higher levels. These flow disruptions have resulted in major water management challenges.
If you are interested in tracking the flow of the Pomme de Terre you can go to the USGS stream gauge website at http:// waterdata.usgs.gov/nwis/uv?site_no= 05294000. Here you can see real time flow data as well as look up historical data since the gauge was established.
Shaun McNally is the Pomme de Terre Watershed Project Coordinator. He is located in the Stevens SWCD office in Morris. The Pomme de Terre River Association maintains a website: www.pdtriver.org