**Course Title: Floodplain Management**

**Module 2:** Stream Systems on Dynamic Earth

**Session 3**: Glaciation & Erosion Processes \-- The Production of
Sediment

**Author**: Dr. Donald R. Reichmuth, P.E. President, Geomax, P.C.
Spokane, WA

(E-mail -- <Geomax@comcast.net>)

**Time 75 minutes**

**Objectives: (PP 1 - 2, Title and Objectives)**

At the end of this session, the students will be able to:

3.1 Summarize how glacial conditions that have existed in North America
for the last 30,000 years affect river basins and floodplains.

3.2 Compare rates and processes of erosion, transport, and deposition.

3.3 Explain the production and movement of sediment by the
agents/processes of erosion \-- water, ice, gravity, and wind.

3.4 Know case study team assignment and discuss glaciation and erosion
processes within the context case study floodplains.

**Scope:**

Virtually every stream in North America was affected by the last
glaciation and is still in the process of reaching a preglacial
equilibrium. To reach a new equilibrium, streams often erode their
channels; aggrade their channels and/or change their course. Each of
these changes can have a profound impact on floodplain stability. This
session explores the relationship that exists between this last
glaciation and the erosion processes that now are active. **(PP 3,
Glaciation & Surface Processes)**

Case study team assignments will be determined during this session.

**Readings:**

***Student and Instructor Reading:***

[General Overview]{.underline}

Pidwirny, M. 2004. *Fundamentals of Physical Geography*, Chapter 10 --
Introduction to the Lithosphere. On line at:
(http://www.physicalgeography.net/home.html)

***Additional Instructor Reading:***

Cronin, T.M. 1999. *Principles of Paleoclimatology: Perspectives in
Paleobiology and Earth History,* Columbia Univ. Press (Especially Ch.
4 - Orbital Climate Change, Ch. 6 -- Holocene Centennial & Decadal
Climate Variability, and Ch. 8 -- Sea Level Change)

Sverdrup, K.A., Duxbury, A.C., & Duxbury, A.B. 2003. *An Introduction to
the World's Oceans*, 7^th^ Edition, McGraw-Hill.

**General Requirements:**

The instructor needs to make sure that the student teams are organized
internally, that the assigned individual responsibilities are clear, and
that each team has begun doing their research. The written work plans
from each team are now due and should be turned in and checked by the
instructor before Session 4 begins.

**Objective 3.1 Summarize how glacial conditions that have existed in
North America for the last 30,000 years affect river basins. (PP 4,
Northern Hemispheric Glacial Areas)**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides.

**Remarks:**

I.  Two types of glaciers have impacted North America in the last 20,000
    years. (PP 5, Pleistocene Glaciation)

    A.  Continental Glaciers covered much of the Canadian Shield and
        North American Platform (including all of the Great Lakes
        Region). (PP 6, Glacial Impact Areas)

        1.  The ice reached a thickness of about 3000 meters (9000 feet)
            in the Hudson Bay Region.

        2.  Many large lakes formed along the southern ice margin.
            Considerable deposition of fine-grained material occurred in
            these lakes.

            a.  Streams easily erode these lakebed deposits because the
                material is unconsolidated.

            b.  Lake Agassiz is an example of an ice marginal lake that
                covered a large part of the Dakotas.

        3.  Many streams were forced to flow in different directions
            because the preglacial drainages were blocked by ice.

            a.  The Upper Missouri River used to flow to Hudson Bay but
                was diverted into the Mississippi River System.

            b.  The preglacial Teays River located south of the Great
                Lakes was largely filled by glacial debris and the area
                is now drained by the Ohio River that was constructed
                from bits and pieces of other preglacial river systems.
                (PP 7, Pre glacial Teays Basin)

    B.  Alpine Glaciers were found in many of the mountain drainages in
        western North America.

        1.  The glacial valleys normally were heavily scoured into
            U-shaped valleys.

        2.  The upper parts of the valleys were over-steepened causing
            considerable erosion to now take place.

        3.  Terminal and lateral moraines formed at the sides and ends
            of the glaciers. Often lakes form above terminal moraines
            that provide more constant downstream flow. The material
            eroded by alpine glaciers often chokes streams with an
            excessive sediment load.

        4.  The Rocky Mountain region was considerably wetter during
            glaciation.

            a.  Many lakes formed in closed basins in Utah, Nevada, and
                surrounding areas. (PP 8, Western Glacial Lakes)

            b.  The Great Salt Lake in Utah is the remnant of Lake
                Bonneville, the largest, that at its peak drained into
                the Snake River in southeastern Idaho.

II. When the glaciers retreated, large areas of unvegetated terrain were
    exposed.

    A.  Glacially derived features such as kettles, drumlins, and
        moraines were exposed that altered the preexisting drainage
        systems.

    B.  Large amounts of wind blown silt (loess) were blown off these
        areas.

        1.  Areas previously covered by ice were a major source.

            a.  Nebraska contains deposits up to 200 feet thick derived
                from sediment produced along the Rocky Mtn. Front.

        2.  Glacially generated sediment deposited on floodplains was a
            source.

            a.  The Columbia River Valley has significant deposits.

            b.  Large deposits occur east of the Missouri and
                Mississippi Rivers. (PP 9, Loess)

        3.  Silt derived from the Lake Missoula Floods has covered a
            large area called the Palouse in eastern Washington and the
            panhandle of Idaho.

        4.  Streams easily erode loess.

            a.  Stream banks are normally vertical.

            b.  If vegetation is removed or the banks sloped, new
                erosion usually follows.

III. The weight of the ice mass caused the Lithosphere to be pushed
     downward into the Asthenosphere and later to rebound when the ice
     mass melted.

     A.  Hudson Bay has rebounded about 100 meters (300 feet) since the
         last glacier in the region started to melt around 16,000 years
         ago. (PP 10, Glacial Rebound)

         1.  The rebound has created a vast complex of abandoned
             shorelines surrounding the bay.

         2.  Numerous rivers in Central Canada have been blocked and
             dammed by the uplift.

         3.  The land surrounding the Great Lakes is now tilting. (PP
             11, Great Lakes Glacial Rebound)

             a.  The North Shore of Lake Superior is rising at a rate of
                 0.5 meters (1.75 feet) per 100 years.

             b.  Southern Lake Michigan is static.

IV. Melting glaciers and warming the oceans cause sea level to rise. (PP
    12, Sea Level Changes)

    A.  At the last glacial maximum 16,000 years ago, ocean levels were
        about 120 meters lower than present.

    B.  Most Northern Hemisphere glaciers, except Greenland, had melted
        by about 6,000 years ago and the oceans reached their present
        level. (PP 13, Holocene)

    C.  Global warming and complete melting of ice in Greenland and
        Antarctica would cause sea level to rise about 70 meters above
        current level.

        1.  Large human populations presently live at elevations10
            meters or less above the current sea level.

        2.  Even moderate global warming and ice melt would cause
            massive human dislocations and huge economic cost.

**Supplemental Considerations:**

The instructor should identify what impact (if any) glaciation had on
local streams and floodplains.

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**Objective 3.2**

**Compare rates and processes of erosion, transport, and deposition.**

**Remarks:**

I.  **Erosion and deposition involve multiple steps.**

    A.  First, tectonic action triggers erosion by rising rock
        (consolidated material) above sea level.

    B.  The second step in erosion is breaking the rock (consolidated
        material) into smaller pieces.

    C.  Then, the liberated material is transported to different
        locations.

    D.  Finally, the transported material is deposited either on the
        earth's surface or under water.

    E.  Often Steps "C" and "D" above are repeated multiple times.

        1.  Each new movement requires new energy or a changed
            environment.

        2.  Steps can occur at various time intervals from a few years
            to thousands of years.

        3.  Ultimately, the transported material reaches a stable site
            (an ocean basin for example).

II. **The resulting erosion rates are influenced by many factors. (PP
    14,Erosion in River Basins)**

    A.  Ongoing tectonic activity can increase erosion rates.

    B.  The original strength and chemical stability of the uplifted
        material influences the rate of breakdown.

    C.  The River Basin Gradient determines the energy available to move
        material.

    D.  Erosion is strongly influenced by climate.

III. **Once exposed, the surface is subjected to attack by other forces.
     (PP 14, Chemical & Mechanical Weathering)**

     A.  This process is called weathering and includes both mechanical
         and chemical processes.

     B.  Freeze-thaw cycles (mechanical) tend to break the uplifted
         material into pieces that can be more readily moved.

         1.  This mechanical action is most common in colder or higher
             elevation environments.

         2.  Individual, separated pieces maintain most of their
             original strength.

     C.  Water and acids (chemical) attack minerals within the material.

         1.  Some grains are dissolved.

         2.  Some grains expand when water is absorbed. This action
             tends to weaken the material.

         3.  Chemical attack predominates in hotter, moist areas.

**Supplemental Considerations:**

The instructor should identify local examples of chemical and/or
mechanical weathering and relate those processes to sediment size found
in the local streams.

**Objective 3.3**

**Explain the production and movement of sediment by the
agents/processes of erosion \-- water, ice, gravity, and wind.**

**Remarks:**

I.  **There are four agents/processes that create and transport sediment
    from bedrock.**

    A.  Rates vary depending on the temperature and precipitation that
        occurs. **(PP 16,Erosion Processes -- Temperature) (PP
        17,Erosion Processes \-- Precipitation)**

    B.  Water is the most common agent of erosion and transport.

        1.  Water enters the molecular structure (hydration) of certain
            rock minerals on exposed surfaces. Upon hydrating, the rock
            minerals expand causing the exposed surface to be torn away
            from the intact rock (exfoliation) leaving an "onion skin"
            appearance. This chemical action greatly weakens the rock
            mass and particles can then be swept away.

        2.  The water entering the molecular structure can dissolve
            certain rock minerals. The dissolved rock can then be
            carried in solution in the flowing water.

        3.  Dissolving minerals containing Calcium (Ca) and Magnesium
            (Mg) often lead to the "hard water" that leaves whitish
            deposits on sinks and faucets. Chemical weathering occurs
            most commonly in moist and humid climates.

    C.  Ice is the most powerful agent of erosion in colder climates.

        1.  Rock can be broken apart by the freeze-thaw action that
            occurs when ice forms in small fractures and joints. This
            mechanical action causes the bedrock to break apart in
            pieces ranging in size from sand to large boulders.

        2.  When stream velocity is sufficiently high, these pieces can
            be mobilized and move downstream.

        3.  Mechanical weathering normally occurs at higher elevations
            and in more northerly areas where cyclic freezing and
            thawing occur.

        4.  In areas where the average temperature is below freezing,
            glaciers tend to form. Moving ice is an extremely powerful
            force. Large pieces of rock can be broken away from bedrock
            outcrops and transported great distances, where later, they
            can be mobilized by flowing water.

        5.  Large quantities of rock are often ground into very fine,
            silt sized particles called rock flour that can be easily
            moved by wind or flowing water.

        6.  Large quantities of debris carried away from a glacier's
            terminal moraine by flowing water often form outwash plains
            or fill valleys.

        7.  Sediment supply associated with glaciation is huge.

            a.  Often river valleys below glaciers become filled with
                sediment.

            b.  After glaciation, the streams often exhume these filled
                valleys and only remnant terraces are left along the
                valley sides.

    D.  Gravity can cause large masses of material to move down-slope if
        there is sufficient gradient.

        1.  Landslides commonly occur on steep saturated hillsides that
            have been either over-steepened and/or contain weak bedrock
            or unconsolidated deposits.

        2.  Debris flows can occur whenever unconsolidated deposits
            become saturated and unstable.

            a.  Earthquakes and volcanic eruptions can trigger debris
                flows.

            b.  Material can move great distances down stream channels
                and cause total destruction.

                1.  For example, the **Osceola Mudflow** carried more
                    than one half cubic mile of material and traveled
                    from the upper flanks of Mt. Rainier all the way to
                    Puget Sound. **(PP 18, Mass Movements)**

            c.  Once these deposits are on the **valley floor**, it is
                easy for streams to entrain the material and carry it
                further downstream.

    E.  Wind is the primary erosion and transport mechanism in certain
        environments.

        1.  Wind borne silt (loess) can be carried great distances and
            be deposited in areas having wetter conditions.

        2.  In dry, hot, climates (deserts for example) wind is the most
            important erosion agent.

        3.  After glaciation, bare sediment covered surfaces can be
            attacked by wind.

            a.  Soon after the last glacial period, large quantities of
                silt (rock flour or loess) were blown off barren outwash
                plains and deposited downwind.

            b.  The Sand Hills of Nebraska, derived from wind borne
                glacial silt from the Rocky Mountain Region, cover more
                than 35,000 km^2^ and averages 8 meters deep.

            c.  The hills east of the Mississippi River contain large
                deposits of glacially derived silt coming off the
                post-glacial river floodplain. Silt deposits form
                vertical banks along streams because slopes are easily
                attacked by rain falling on the fine-grained soils.

II. **The rates of geologic processes vary considerably. (PP 19, Class
    Handout)**

    A.  Deposition in oceans is the slowest (average \-- 0.003 meters
        per 1000 years).

    B.  Surface erosion from entire river basins is intermediate
        (average \-- 0.09 meters per 1000 years).

    C.  Vertical tectonic movements are high (typical \-- 1 meter per
        1000 years).

    D.  Horizontal tectonic movements are higher (typical \-- 10 meters
        per 1000 years).

    E.  Initial glacial rebound is the highest (typical \-- 80 meters
        per 1000 years).

**Supplemental Considerations:**

The instructor should identify the relative importance of the four
erosion processes on local erosion rates.

**Objective 3.4**

**Know case study team assignment and discuss glaciation and erosion
processes within the context case study floodplains.**

**Remarks:**

A.  Student teams should be given time to determine/confirm case study
    team assignments and comment on or ask questions concerning the team
    work plans.

B.  Teams should discuss the production of sediment within the context
    of their case study floodplains**.**

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**References:**

Farrand, W.R. 1998. *The Glacial Lakes Around Michigan*, Bulletin 4,
Geological Survey Division, Michigan Dept. of Environmental Quality, 16
pgs.