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Measuring Knickpoint Migration in Ravine Z, Seven Measuring Knickpoint Migration in Ravine Z, Seven Mile Creek Park, Nicollet County, MN By Michael Dickens A thesis submitted in partial fulfillment of the requirements of the degree of Bachelor of Arts (Geology) at Gustavus Adolphus College 2015 Measuring Knickpoint Migration in Ravine Z, Seven Mile Creek Park, Nicollet County, MN By Michael Dickens Under the supervision of Laura Triplett Abstract The Minnesota River is facing increasing sediment loads, which are a result of sediment erosion in the rivers watershed. Likely sources for that sediment include upland topsoil, incising and head-cutting ravines, Bluffs and streambanks. The focus of this study is ravines, which are poorly understood in terms of erosional processes. One main way that ravines erode is through knickpoint migration, which happens as water flows over a tougher material, and falls onto a softer material, creating a back-cutting and over-steepening effect at the toe of the knickpoint. Material from the Bottom of the ravine is thus moBilized, and can be transported down the ravine into the Minnesota River. To help decipher the role of knickpoint migration in sediment loading on the Minnesota River, we examined a single ravine and its knickpoints over a span of several years. Seven Mile Creek, a tributary to the Minnesota River in Nicollet County, is an ideal location to study the factors that contribute to knickpoint migration. Ravine Z, a prominent ravine in Seven Mile Creek Park, Nicollet County, MN, is a very active eroding channel that is largely fed By farm drainage tiles. A digital elevation model and surveying tools were used to make a series of slope profiles spanning the period 2007-2014. Rates of knickpoint migration could then Be determined and compared to the precipitation record over each time interval. Results of this study indicated knickpoint migration in Ravine Z is rapid, with rates between 10.47-34.27 feet per year on average. These data suggest that Both the amount and intensity of rainfall influence knickpoint migration rate in Ravine Z. 2 Acknowledgements For a project so large, I have plenty of people to thank. First, I would like to thank Dr. Laura Triplett for Being my advisor, and keeping me on track though the year. I would also like to thank Dr. Julie Bartley for her help during J-term and spring semester, as I had many questions to ask during that section of time. I would like to thank Dr. Jim Welsh for providing me with many materials for the use of Better understanding my site. I want to thank my friends, Jeff Halvorson and Dom Delmont, for going into the field with me to collect data, and Scott Hauer and Lance Erickson for providing me with their 2013 data profile. Lastly, I want to thank my parents and family for all of their support. 3 Table of Contents Introduction 5 Geologic Setting 8 Methods 11 Results 13 Discussion 19 Conclusion 23 References 24 Figures Map 1- Map of Minnesota 8 Figure 1- Slope profile correction 14 Figure2- Slope profile 2007-2013 15 Figure 3- Slope profile 2013-2014 16 Figure 4- Slope profile 2007-2014 17 TaBle 1- Precipitation data 17 TaBle 2- Migration rate 18 4 Introduction The Minnesota River is a large river that drains the southern half of Minnesota before it flows into the Mississippi River. The Minnesota River is considered to be impaired due to high suspended sediment and turBidity under the US Clean Water Act (Wilcock et al., 2009). This can cause many proBlems, such as the infilling of different bodies of water that receive water from the river (MPCA 2007). Water clarity also decreases, causing people to not want to Be around the river. Also, it can negatively affect the ecological health of the river. Suspended sediment decreases the amounts of photosynthesis that plants can do, and clog the feeding apparatuses for filter feeders such as mussels (Best et al., 2001; WilBer and Clark, 2001). Sediment erosion in the Minnesota River watershed is a serious concern for people living Both inside and outside of Minnesota. The amount of sediment erosion has increased in the Minnesota River watershed are causing different lakes and rivers that are fed by the Minnesota River, such as Lake Pepin, to Be infilled with sediment (MPCA 2007). Increased sediment loads in the river can cause problems with drinking water. Many people in and out of Minnesota use the Mississippi River, downstream of its confluence with the Minnesota River, as a source of drinking water. When there is a higher amount of sediment, they must have their municipal water supplier use a stronger filter system. The Minnesota and Mississippi Rivers are well known for fishing, But if the rivers have too much sediment, fish can struggle (WilBer and Clark, 2001). 5 Much of the Minnesota River’s sediment load comes one suB-watershed, that of the Le Sueur River in south-central Minnesota. In 2011, the Minnesota Pollution Control Agency created a sediment Budget for the Le Sueur River, in order to better understand how much sediment has Been added by anthropogenic processes on a yearly basis compared to natural amounts (Gran et al., 2011). This analysis determined that sediment in the river comes from a variety of sources in the landscape, such as Bluffs, ravines, or topsoil. Like the Le Sueur River, The Minnesota River gets much of its sediment from Bluffs and ravines, and though the amounts of sediment that the river receives from Bluffs has been well calculated, the sediment contriBution By ravines is still uncertain as ravines can either store or erode sediment (Wilcock et al., 2009). Ravines are relatively short and steep triButary streams that feed water into larger bodies of water, such as creeks or rivers. Ravines incise through the glacial till of the Minnesota River valley, allowing water to drain from the uplands down to the river. Ravines connect the uplands of the watershed, which is a glacial till plain, to the river. When water flows through a ravine, some amount of erosion occurs in the Bed of the ravine. Ravines can also store sediment for periods of time when sediment is deposited in the middle of the ravine when there is a decrease in water discharge (Wilcock et al., 2009). Ravines erode in a combination of hillslope and river processes (Wilcock et al., 2009). If there is a more resistant portion in the ravine, like a tree root or bedrock outcrop, that portion will resist erosion, while the softer part will erode. This causes knickpoints, which are areas in the ravine that have a much steeper slope, and they are of interest because they are the points in ravines that produce a lot of sediment. The knickpoints will then migrate towards the head of the ravine when water is flowing through the system 6 because as water falls over the knickpoint, the vertical face of the knickpoint is eroded more quickly than the rest of the knickpoint. Understanding how knickpoints erode and migrate is very important when looking at an area dominated By agricultural land use like southern Minnesota. Because erosion in ravines only happens during times of water flow, the rates that knickpoints move can change from year to year as precipitation and runoff vary. Knickpoint migration rate depends on the type of geologic material the knickpoint is eroding through, the amount of water that enters the ravine, and the time that the knickpoint has to erode. Although this part of the state has Been “primed” for increased amounts of sediment erosion Because of the soft unconsolidated till unit that forms the valley, modern agricultural methods have increased the rate of erosion of sediment into the Minnesota River even more (Belmont et al, 2011). Approximately 65% of the Minnesota River watershed is used for farming annual row crops (Wilcock et al., 2009). That cropland is often drained By a system of drain tiles, which are used to decrease the amount of standing water has on cropland. Ravines are important to study when looking at sediment erosion into the Minnesota River Because many drain tile outlet pipes flow into ravines. Drain tile systems often have an outlet at the top of a ravine, and contriBute a large amount of water to ravines. That water increases the amount of erosion in the ravine, which in turn may lead to higher suspended sediment in the Minnesota River. 7 River processes are mainly controlled By knickpoint migration in the ravine. As a knickpoint migrates, hillslope erosion happens as the ravine walls oversteepen and slump, which widens the ravine (Gran et al., 2011). Here, I used a combination of field oBservations, digital elevation model (DEM) analyses, and precipitation data to explore knickpoint migration rates, as well as to determine what seems to play the Biggest factor in determining the rates of knickpoint migration. 8 Geologic Setting Southern Minnesota is largely covered by Wisconsin age till, though there are outcroppings of other geologic units in some of these areas. The Minnesota River drains this section of the state, and it flows into the Mississippi River on the southeastern portion of the state. The Minnesota River is fed by a series of ravines and creeks that drain water from the uplands into the river. My field area is Seven Mile Creek Park in southeastern Minnesota, near Mankato, MN in Nicollet County. Map 1- This shows the regional and study area. Seven Mile Creek Park is highlighted in green on the study area map, and the creek is highlighted in red. Map taken from Jeremy Bock, 2010. The geology of Seven Mile Creek Park consists of Jordan Sandstone, Oneota Dolostone, and glacial till.
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