Over 12,000 years ago much of the Nemadji River Basin, including Deer Creek, was submerged beneath Glacial Lakes Duluth and Nemadji. The lakes drained and left behind glacial lake (lucustrine) deposits of clay, silt, and very fine sand. Additionally, glacier ice deposits left behind glacial clay till. Today, approximately one half of the Deer Creek Watershed is comprised of glacial till and lake laid clay soils. The map below depicts the most recent glacial phases of Carlton County.
The complex geological setting of the Deer Creek Watershed and greater Nemadji River Basin has been studied by many researchers. The geological history of the area helps researchers understand current hydrological behaviors in the watershed. Geomorphology (shape of the land) is closely linked to the understanding of the hydrology (movement of water). Stream water pathways through land are responsible for sediment transport, including erosion and deposition. Additionally, groundwater pathways to the stream can contribute to sediment loading in a stream channel, which is the case in Deer Creek. The locally known “Mud Volcanoes” contribute sediment to the stream via groundwater.
Geomorphology and Physical Channel Assessment
While the turbidity TMDL study ultimately seeks to define the sediment load of the stream, the broader system functioning of the stream and watershed must be considered. A geomorphology and channel assessment investigation will provide valuable information about processes such as erosion and slumping.
A stream channel is formed through erosion and deposition. High flows are responsible for doing the work of channel formation and maintaining the channel over time. Historical land use changes have likely increased the water yields in the system. This influx of water has caused more downcutting and incision in the stream channel causing more sediment loading to to the system (Reidel, Verry, Brooks 2002).
Tree Splits such as the one displayed below, are a result of a rotational slump. Half of the tree roots were taken down slope with the land, while the other half remained at the original location. The continued movement of the land down slope held the tree roots and caused the tree to topple upslope. These types of slumps are common in the lucustrine clay portion of the Deer Creek watershed. The map below displays a slump inventory was completed in 1994. Slumps over 25 square feet in size were identified in the stream valley of Deer Creek through photo interpretation and field checks. The map also displays the location of the mud volcanoes, which form along the slump fault scarps.
Groundwater and the Deer Creek Mud Volcanoes
Sediment contribution via groundwater can be responsible for high sediment yields, even during baseflow conditions. Recent studies have analyzed such occurrences in Deer Creek and will be apart of the TMDL study.
2005-University of Minnesota-Duluth
A groundwater seepage investigation was conducted along a reach of Deer Creek in 2005 by researchers from the University of Minnesota-Duluth. The study concluded that groundwater from sand confined aquifers moves through fault scarps in lacustrine clays and clay till sediments bringing sand to the surface. These discharge points, called “sand volcanoes” or “sediment volcanoes,” were found to be significant contributors of sediment in Deer Creek. The study suggested that the shear strength of the clay and the driving shear stress are very close to one another, leading to rotational failure when the shear strength is reduced, possibly due to increase in pore pressure. When the faults begin to form due to rotational failure, dewatering occurs, carrying along aquifer materials. The study indicated that the phenomenon is widespread throughout the region (Mooers and Wattrus, 2005). A U of M Duluth graduate student is currently conducting a follow up study to the 2005 groundwater seepage investigation on a broader scale in the Minnesota protion of the Nemadji River Watershed. Initial results have indicated that sheer strength is high in areas of low slope and sheer strength is low in areas of high slope. Slope failure was generally found to be susceptible in river valleys. Of 41 suspected slump areas, 21 slump areas have been verified.
Deer Creek Groundwater Model
The MPCA recently completed a groundwater model for a portion of Deer Creek watershed that includes the sand/mud volcano area. Results of the model have indicated a connection between the thinning of the clay layer and likeliness for surficial groundwater discharge. Further modeling will investigate the reduction of groundwater seepage through the establishment of new vegetation. The model will explore the idea of tree root interception of water discharge to streams through data collected on transpiration.
Stable Isotope Study-The use of stable isotope analysis of 18O and Deuterium will assist in delineating hydrologic pathways of the sediment sources. Groundwater has a uniquely different isotopic composition than water from stream channels and precipitation. Samples will be taken from each of these hydrologic sources and analyzed for their isotopic composition. Once these different compositions are determined, the source and quantity of water can be allocated within the stream.