Geomorphology is the study of landforms; from their origin and evolution to the processes that continue to shape them. The term is derived from the Greek geo, meaning earth, and morphe, meaning form. Geomorphologists seek to understand landform history and dynamics, and predict future changes through a combination of field observation, physical experiments, and modeling.
Landforms evolve in response to a combination of natural and anthropogenic processes. Large-scale geologic forces such as uplift, volcanic activity, and glacial erosion and deposition shape the land over which rivers eventually flow. Geology sets the stage for what is to come.
The shape of each stream reacts to certain variables in predictable and measurable ways. For example, the natural form of the low gradient streams characteristic of most of Minnesota is sinuous, narrow, and deep. Steeper rivers found on the North Shore of Lake Superior are less sinuous and have boulder and bedrock rapids more like mountain streams. Many variations on these forms can be found throughout the world.
Large-scale human activity is also a force changing landforms and impacting landscape-level processes. Building dams and dikes, converting existing land cover to crop production, and adding impervious surfaces are examples of human induced change with geomorphic force. These changes impact and alter the other natural geomorphic processes occurring across the landscape.
Explore the Geomorphology Health Scores to see a series of index values that show health trends in the geomorphology of Minnesota.
Fluvial geomorphology focuses on the dramatic hydrodynamic forces that shape rivers; the result of the interplay between the force of moving water and the materials forming the streambed. Hydrodynamic forces form islands and sandbars, grade rapids and riffles, create deltas, and form—and eliminate—meanders. Therefore, rivers and streams are not only conduits of water, but also of sediment. The water, as it flows over the channel bed, is able to mobilize sediment and transport it downstream. As water moves through a meandering channel, changes in streamflow cause the sediment to be scoured, sorted, and deposited in riffles and pools (Minnesota DNR, Healthy Rivers).
There are eight variables determined by climate and geology that interact to create the form of a stream channel (Leopold et al. 1964; Heede 1992; Leopold 1994):
- Discharge (the volume of water)
- Sediment supply
- Sediment size
- Channel width
- Channel depth
- Water velocity
- Slope (or gradient)
- Roughness of channel materials
These variables reflect the importance of interactions between the stream and the watershed landscape of bedrock, topography, soils and climate within which it lies. These variables interact in predictable and measurable ways. For example, the natural form of the low gradient streams characteristic of most of Minnesota is sinuous, narrow, and deep. Steeper rivers found on the North Shore of Lake Superior are less sinuous and have boulder and bedrock rapids.
Healthy streams are able to carry a certain amount of sediment over time in a sustainable balance. With the addition of excessive sediment (soil erosion from farmland, for example), the stream will deposit excess sediment in the channel as riffles, bars, or islands. A dramatic reduction in sediment, such as construction of a dam that traps sediment will cause the downstream channel to enlarge by widening and down cutting.
The relation between discharge and sediment transport is important because the complex interactions between these elements determines whether the stream channel is stable, aggrading or degrading (Lane 1955).
Bankfull flow is generally defined as the height of the floodplain surface or the flow that "just fills the stream to its banks" (Gordon et al. 1992; U.S. Forest Service 2003) or the stage at which water starts to flow over the floodplain (Dunne and Leopold 1978). Bankfull flow is subject to minimum flow resistance (Petts and Foster 1985) and produces the most sediment transport over time (Inglis 1949; Richards 1982). Bankfull events have a recurrence interval of approximately 1.1-2.0 years. Scour of fines from pools and deposition of bedload in riffles is also most predominant at bankfull flow, creating and maintaining high quality habitat. (Leopold et al. 1964; Mosley 1981; Rosgen 1996).
The free play of a stream’s hydrodynamic forces creates a rich diversity of habitats for aquatic life. This diversity results from variation in stream depth, width, water velocity, and substrate throughout the channel, (the antithesis of a channelized ditch). Riffle habitat provides feeding locations for many species of fish and spawning habitat for species such as walleye, lake sturgeon, trout, darters, and suckers. Many aquatic invertebrates rely on riffle habitat. Pools and eddies provide spawning and feeding areas for species such as smallmouth bass, and provide some invertebrate species with slow water and finer sediment.
It is important to recognize that the physical habitat essential to maintaining the aquatic community is formed by periodic disturbance. This disturbance in the short-term may be detrimental to the growth and survival of individual fish or entire year classes of fish. On the other hand, high flows reset the system. Formation of new channels, scouring vegetation and fine sediments, abandoning side channels; all work to create habitat beneficial for some species over the long-term. Periodic resetting of the system is an essential process.
The interactions between the variables that create a stream channel are complex and multi-dimensional. Continued study of these interactions is essential to understand the consequences that intended and unintended alterations can have on the health of the entire watershed system.
Stream Health Resources
Considered in geologic time, Minnesota’s landscapes are dynamic and constantly changing. Long before historic human occupation, drastic changes occurred when massive sheets of ice pushed across the state. As these sheets of ice inched southward, growing as snow accumulated, they shaped Minnesota’s four hydrologic regions.
When the glacial lobes began their retreat around 14,000 years ago, the resulting meltwater formed enormous rivers and lakes. The largest of these, Glacial Lake Agassiz, with a basin of almost 600,000 square miles, covered all of northwestern Minnesota at one time and was the largest glacial lake in North America.
Lake Agassiz began forming in the southern Red River valley 11,700 years ago and finally disappeared from the state around 9,000 years ago. During much of this period, the lake’s northern outlets were barricaded by ice. Thus, its only outlet was the Glacial River Warren, which drained to the south and whose river corridor is visible today as the broad Minnesota River valley. As the ice continued to retreat, previously blocked northern drainage outlets gradually opened, and Lake Agassiz began to drain northward, as the Red River does today (Minnesota DNR Tomorrow's Habitat, 58).
The Red Lakes and Lake of the Woods in Minnesota, as well as some large lakes in south central Canada, are remnants of the much larger Glacial Lake Agassiz. This geologic history influenced the landscape in ways that are still very evident. The slightly elevated sand and gravel beach ridges in east central Minnesota, and the flat plain of silt and clay soils through which the Red River now runs, are landscape inscriptions from an ancient lakebed.
Minnesota’s geologic history is also evident in the soils and plants that create today’s landscapes. The Ecological
Classification System (ECS) uses biotic and environmental factors such as climate, geology, topography, soils, hydrology, and vegetation to map, study and classify ever more detailed levels of biological associations.
As a broad example, the Eastern Broadleaf Forest (EBF) Province covers nearly 12 million acres of the central and southeastern parts of the state. The land surface is largely the product of the Pleistocene glacial processes.
The northwestern and central portions have deposits 100-300 feet deep of glacial drift. Huge volumes of meltwater from glacial lakes cut deep valleys along the present course of the Minnesota, St. Croix, and lower Mississippi rivers. The southeast, which was not covered by ice in the last glaciation, experienced headward stream erosion as the Mississippi River valley deepened. This exposed Paleozoic bedrock creating its characteristic bluffs. In the Twin Cities area, channels of pre-glacial rivers cut through rock formations, which later filled with glacial till. Once the till settled, the chains of lakes that now meander through the Twin Cities formed in the depressions.
A wide variety of plant and animal species have adapted to this diverse array of landscape features. And this province is home to the majority of Minnesotans. Human induced changes through activities such as road building and row crop agriculture have become a very significant geomorphic force in more recent times. Managing this rich biological heritage while balancing the needs of an increasing human population will be a long-term challenge.