To log this Earth Cache, please answer the following questions
Look around at the base of the gully where it connects to the South Saskatchewan River for any evidence of erosion. Look for any sedimentary deposits that may have washed from the gully, if any. The following is NOT a required EarthCache question to answer, and is only a geology research adventure if you are very careful on a hilly climb/descent; What is the approximate length of the gully from the head to the base by using your GPS, or geocaching app? (For geotechincal reports, geologists measure the variation of gully length from year to year)
For this EarthCache please answer the following questions:
1. Does the evidence observed near either end of the gully tell you this gully is stable, or is the erosion process still active? (Why?)
2. If these gullies continue to enlarge and contribute to soil erosion, how will they impact the surrounding land?
3. What can you do to help stabilize gullies? How would a property owner’s approach contribute to gully stabilization given their choices of maintaining a Kentucky bluegrass lawn or replacing it with xeriscaped native pollinator plantings? How would conserving electricity over the colder winter months play a role? (Refer to the "Schematic diagram of gully head"; vis à vis scouring and eddies)
4. Take a picture of yourself, or your GPSr with the gully in the background (face not required) You may provide the photo in your earthcache geocaching log of your visit or identifying feature showing you were here at Ground Zero, or at some point along the erosional feature of the gully.
5. Now for fun! What is your favorite pastime when observing and visiting Diefenbaker Park?
DO NOT POST ANSWERS IN YOUR LOG. Please don’t provide the answers when logging the cache online except for the photo. For the queries use the “Send answers” feature OR geocache mail the cache owner including the earth cache GC number, title and the answers. 
Please answer to the best of your ability. As long as you give it your best effort, we'll be happy to accept your responses. You will probably the answers you are looking for in this description page!
Saskatoon City Transit Route 1 bus stop 3621 is located at St. Henry and Ruth Street; a 1.5 km or 20 minute walk to GZ.
EarthCache: Bifurcated Gully at Diefenbaker Park –
Understanding Gully Retreat, Erosion, and Erosion Control Techniques
Welcome to the Bifurcated Gully located along the east riverbank of the South Saskatchewan River in Diefenbaker Park, Saskatoon. This EarthCache will guide you through the processes of gully retreat, the natural forces that drive soil erosion, and various schemes to stabilize and prevent gully expansion. We will also explore various erosion control methods that help mitigate the impact of gully erosion, ensuring that these fragile ecosystems are protected. Did you know? Diefenbaker Park is ranked #10 on Saskatoon's "Top 10 Nature Watching Sites"! 🌿🦅 and a great hot spot for geology and earthsciences study.
Definition and Formation of a Gully
A gully is a landform created by running water, eroding sharply into soil, typically on a hillside. Gullies resemble large ditches or small valleys and can vary in size, length, depth, and width. The formation of a gully begins when water flows down a slope, eroding the soil and creating a channel. When the water flow rate is substantial, the cutting action can be deep and dramatic, removing large amounts of soil. As this process continues, the gully expands, becoming more pronounced over time.
Gullies are typically related to intermittent or ephemeral water flow, which occurs due to intense rainfall or snowmelt events. In areas with insufficient groundcover (due to deforestation, overgrazing, or human development), the erosion process is accelerated as water runs freely, removing soil and sediment from the surface. This action is known as erosion, a process in which the earth’s surface wears away due to natural forces like water, wind, glaciers, or storms. The eroded soil is carried away by the water, often forming sediment that can be smoothed by continued runoff.
Eventually, when the water flow rate decreases, and no more erosion takes place, the gully becomes stable. At this point, no further increase in the size of the gully (its width, length, or depth) occurs. However, before this stabilization happens, gullies may expand dramatically, carving deep channels into the earth’s surface. This is what makes gullies unique: they begin as small depressions or ditches but can evolve into substantial landforms due to the ongoing action of water.
Geological Context: The Formation of Gullies
In Diefenbaker Park, the bifurcated gully is split into two distinct branches, the east gully and the west gully, a result of natural erosion processes over time. Wooded slopes existed before urban development, but alterations in drainage patterns due to the construction of an urban park around 1973 have led to significant gullying. The retreat of the gully headcuts has been actively measured and observed, revealing significant changes in the landscape, with the headwalls retreating by approximately 7 and 19 meters between 1994 and 2000.
Erosion observations
Being mindful of your surroundings while observing erosional processes at the head and base of a gully with intertill stratified drift (sandy and clayey layers) will help the geocacher identify several key signs of erosion:
At the Gully Head (Top of the Gully):
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Visible Cracks or Shearing: The gully head may have visible cracks or chunks of soil that appear to have fallen away or slumped. This happens because water undercuts the soil, weakening the structure and causing it to collapse. The cracks may widen over time, leading to further erosion.
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Retreating Edge: The edge of the gully, where it begins, may be gradually moving further back into the landscape. A layperson might observe that the gully head is not stationary but is slowly eroding upstream, creating a larger and deeper channel. There could be exposed sandy and clayey layers that appear loose or unstable.
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Wet or Saturated Soil: After rainfall or snowmelt, the soil at the gully head might appear saturated or muddy, with water pooling or flowing over the surface. This is a sign that the water is actively eroding and carrying away the soil, especially in areas where the sandy soil is easily washed away.
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Loose Soil: The soil may appear loose and crumbly, especially in areas where the clayey and sandy layers are exposed. These types of soils are more easily eroded by water, and a geocacher might notice the soil is not tightly packed, leading to further erosion when it rains.
At the Gully Base (Bottom of the Gully):
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Eroded Channel or Ruts: The base of the gully might show signs of active erosion, such as deepening or widening channels where water has flowed over time. There may be visible ruts or depressions in the soil caused by the water’s force, indicating that the water has been actively scouring the area.
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Sediment Deposition: The geocacher might notice areas where sediment has built up along the bottom of the gully. This could appear as piles or layers of sand or fine clay particles. When the water slows down, it drops the eroded soil it has carried, creating new deposits at the gully's base.
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Pooling Water or Small Puddles: In the bottom of the gully, especially after rainfall, there might be small pools or puddles of water that have collected in the eroded areas. These pools can contribute to the erosion process by slowly wearing away the soil.
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Plunge Pools or Small Waterfalls: If water is flowing over the edge of the gully, the geocacher might observe small waterfalls or plunge pools at the base. The falling water can erode the soil, forming small depressions or holes, which further destabilize the base of the gully.
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Wet and Undercut Areas: At the base, there may be areas where the soil has been undercut by water, leading to overhanging soil or unstable edges. These areas are prone to further collapse, and a layperson may notice that parts of the gully floor appear eroded and unsupported.
In summary, in the field, a geocacher observing a gully with intertill stratified drift would notice signs of erosion such as cracked or slumped soil at the top, deepened channels or ruts at the bottom, wet and loose soil, and areas where sediment is deposited. These observable signs indicate the ongoing erosional processes, where the sandy and clayey layers make the soil more susceptible to water-induced erosion.
What is Gully Headcut Retreat (GHR)?
Gully Headcut Retreat (GHR) refers to the process by which the headwall of a gully (the upper portion where the gully begins) moves upstream. This occurs due to erosion at the base of the headcut, where water undercuts the soil and causes collapse. As the headcut retreats, the gully widens and deepens, reshaping the land over time. This process is particularly active in areas where seasonal precipitation, freeze-thaw cycles, and subsurface water flow are prevalent, as seen here in Diefenbaker Park.
In urban areas like Saskatoon, GHR is compounded by human activities such as the construction of storm sewers, which alter drainage patterns, and land development that increases runoff. The gully’s retreat can be particularly significant, if it poses risks to infrastructure, including nearby roads, municipal services, and private properties. In this case the GHR would affect Diefenbaker Park. Both the City of Saskatoon Engineering Department and the Meewasin Land Trust undertake riverbank stabilization projects which include monitoring, management, and restoration of slope stability.
Processes Driving Gully Retreat
Several geological processes contribute to gully formation and retreat:
- Tension Cracks: As soil becomes saturated and unstable, it forms cracks along the gully walls, which, under tension, lead to further erosion and undercutting.
- Piping: Subsurface erosion, or "piping," occurs when water flows through the soil and creates underground channels that weaken the gully banks. This often happens in areas with intertill stratified drift, where layers of sandy and clayey deposits make the soil more prone to erosion.
- Plunge Pools: Water falling over a steep headcut creates a plunge pool at the base. The force of the falling water erodes the soil further, weakening the headwall and accelerating the retreat.
- Freeze-Thaw Cycles: The cyclical freezing and thawing of water in the soil during winter and spring can exacerbate erosion, especially in the presence of moisture.
*Schematic diagram of gully head -> Something to contemplate: How would the scouring caused by the eddies formed underwater in the South Saskatchewan River impact the riverbank's shore and soils under the following conditions: 1) When water is released from the hydroelectric facilities at Gardiner Dam and downstream to meet peak power demands, especially during the winter? and 2) When 10 inches of solid ice prevents water volume expansion to accept the faster flow and water increases between the shorelines of the rivervalley.
Techniques for Gully Erosion Control
To address the ongoing erosion, several gully erosion control techniques have been developed. These methods aim to slow down or stop the retreat of gully headwalls, stabilize the soil, and encourage vegetation growth to provide long-term protection. A few of these techniques include:
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Log or Plank Check Dams:
- These are used in gullies with large volumes of runoff. The dams slow the flow of water, and help to dissipate the energy of the flowing water, allowing sediment to settle and stabilize the gully.
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Single Post Brush Dams:
- A temporary structure, the single post brush dam is installed long enough for vegetation to take root and stabilize the gully. These are particularly effective in smaller gullies with low to moderate runoff.
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Double Post Brush Dams:
- This is similar to the single post brush dam but is used when the energy levels of the flow are too high for a single post structure to withstand. The double post design (also temporary) provides additional support to resist the erosive forces and helps stabilize the gully long enough for vegetation to establish itself.
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Brush Checks:
- Brush checks are suitable for gullies with minimal or small amounts of runoff. Brush checks involve placing branches or shrubs across the gully to slow water flow and trap sediment. This method is effective in controlling erosion in shallow, smaller gullies.
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Brush Paving:
- Brush paving stabilizes gully channels, particularly on steep slopes with considerable runoff. This technique involves placing brush and vegetation along the gully to create a temporary barrier that helps slow water and supports vegetation growth. It is especially effective on steeper slopes but requires the gully bottom to be rounded or flattened, and the gully banks sloped to a minimum of 1:1 to withstand peak flow periods.
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Gully Head Erosion Control:
Methods of controlling gully head erosion focus on altering the catchment area to minimize or redirect runoff. This helps stabilize the gully's head, floor, and walls through the use of various approaches, such as building structures, conducting earthworks, planting vegetation, or installing fencing.
Re-Vegetation and Long-Term Maintenance
Sparse ground cover is one of the causes of gully erosion. As part of the erosion control process, vegetation plays a crucial role in stabilizing the gully. Native grasses, forbs, and trees are especially important for slope stability, as they have deeper root systems that provide better soil cohesion. However, the introduction of invasive species can create monocultures that weaken the soil and exacerbate erosion. Species such as European Buckthorn and Kentucky Bluegrass, both shallow-rooted, may increase bank instability, furthering a cycle of erosion and slumping.
The long-term effectiveness of gully erosion control depends on swift revegetation and ongoing maintenance. Weed control and the encouragement of native species are essential to preventing further erosion and ensuring that the gully remains stable for the future.
Conclusion
The bifurcated gully in Diefenbaker Park provides a fascinating study of gully retreat and the erosion forces that shape this landscape. By understanding the processes of gully formation and the techniques used to stabilize these dynamic features, we gain insights into the delicate balance between natural processes and human interventions. Through methods like log check dams, brush paving, and gully head erosion control, geotechnical engineers and geologists can mitigate the impact of erosion, prevent infrastructure damage, and ensure the long-term stability of the landscape.
Glossary of terms:
A catchment area (also referred to as a watershed or drainage basin) in the context of a gully is the land area that drains into the gully through surface runoff, groundwater flow, or both. It is defined by the topographic boundaries, often characterized by ridgelines or higher elevation areas, that direct water toward the gully. The catchment area collects precipitation and meltwater from rainfall, snowmelt, or other sources, which then flow over the landscape, converging into the gully. The characteristics of the catchment area, including its size, slope, soil type, vegetation cover, and land use, significantly influence the volume and speed of water runoff, erosion, sediment transport, and deposition within the gully. Proper understanding of a gully's catchment area is crucial for studying erosion dynamics, water flow patterns, and the long-term evolution of the gully system.
A declivity refers to a downward slope or inclined surface of the land, often describing a gradual or steep descent in terrain. In the context of gullies, a declivity is the sloping area that leads to the formation of a gully, where surface water often begins to concentrate and flow, contributing to erosion and the development of the gully.
Drift refers to a collective term for all types of sediment—such as clay, silt, sand, gravel, and boulders—that are transported by a glacier and deposited either directly by the ice or by glacial meltwater. This sediment can be found in areas formerly covered by glaciers and is typically classified into two main types: till, which is deposited directly by the glacier, and outwash, which is carried and deposited by meltwater from the glacier. Drift is a key feature in understanding the landscape and sedimentary processes associated with glacial activity.
- Eddies (in Fluid Mechanics/Geology):
Eddies are circular or spiral movements of water that occur when the flow of a river or stream is interrupted by an obstacle, such as rocks, bends, or changes in velocity. These localized whirlpools create regions of slower or reversed flow, where particles of water and sediment may accumulate. Eddies play a role in sediment transport, deposition, and the overall dynamic behavior of river systems.
A flume is an artificial or natural channel often lined with materials like concrete or rock, designed to convey water and simulate natural river or stream conditions, typically designed to carry water over a specific path, often constructed to control water flow or measure its speed and volume. In geological and engineering contexts, flumes are used for studying hydraulic and erosion processes in watercourses, including gullies. A flume can also be used in erosion control techniques, where it serves to direct water flow away from critical areas or stabilize gully headwalls by controlling water velocity and preventing further erosion.
The physical weathering process in which water enters cracks in rocks, and upon freezing, expands and exerts pressure on the rock. This repeated freezing and thawing can cause the rock to fracture and break apart over time.
A gully is a landform characterized by a deep, narrow channel or ravine, typically formed by the erosive action of water. Gullies are a result of erosion processes, where water flow, especially during heavy rainfall or snowmelt, carves into the soil, causing the ground to retreat and the channel to expand in both depth and width. Gullies often develop on hillsides or sloped terrain where runoff concentrates, leading to significant erosion of the surface soil and sediment transport. Over time, a gully may become more pronounced as the water continues to erode the soil, sometimes forming steep-sided valleys or ditches.
The process by which the head of a gully (the upstream part) migrates upstream due to erosion, often exacerbated by water flow, reducing the stability of surrounding soil and leading to further erosion. This can cause the gully to expand in size and depth.
The steep or sloping sides of a gully, formed by erosion processes, that define the boundaries of the gully. The sidewalls are typically subject to ongoing erosion due to water runoff, freeze-thaw cycles, and other weathering processes, which can cause them to become unstable over time.
- Headcut (in relation to Gullies):
A headcut is the uppermost, often vertical or steep section of a gully, where erosion is most pronounced. It is the point where the gully begins to expand upstream, typically due to water flow undermining the material at the gully's head. The headcut progressively moves upstream over time as erosion deepens and enlarges the gully, leading to the retreat of the gully’s head.
The steep, often vertical, upper part of a gully or ravine, typically formed by erosion. It represents the source or "head" of the gully, where the erosion processes are most active.
- Intertill Stratified Drift (in relation to gullies):
Intertill stratified drift refers to sediment deposits that occur between different layers of glacial till. These deposits are typically sorted and stratified, meaning that the particles are arranged in layers according to their size, with finer materials like sand and silt separated from coarser materials like gravel and cobbles. In the context of gullies, intertill stratified drift can influence erosion patterns, as the more sorted, permeable layers (like sand) may erode more easily than the compact, less permeable till layers, potentially affecting the stability of gully sidewalls or floors.
- Piping (in relation to gullies):
Piping is a type of subsurface erosion process in which water flowing through soil or sediment creates small tunnels or channels, often underneath the surface. In the context of gullies, piping occurs when water erodes the soil from beneath the gully sidewalls or bed, potentially weakening the structure and causing collapse or further erosion. This process typically occurs in porous, unconsolidated materials and can contribute to gully enlargement and retreat.
A Plunge pool is a depression or basin-like feature found at the base of a waterfall or gully headcut, created by the erosive action of falling water. As water plunges over the edge, it gains velocity, and the force of the water erodes the underlying material, typically rock or soil. Over time, this erosion deepens and widens the pool, contributing to the further undermining of the surrounding gully walls or waterfall base. Plunge pools are typically associated with processes like hydraulic erosion, where water's kinetic energy erodes soft or cohesive substrates, contributing to gully expansion and headcut retreat.
A dense layer of intertwined plant roots, often found in forested areas or wetlands, that helps stabilize the soil. It provides structural support for the ground, preventing erosion and contributing to soil retention.
Scouring refers to the process of erosion caused by the force of flowing water, which removes soil, sediment, or rock from the riverbed, banks, or other surfaces. Scouring occurs when the velocity of water increases, often due to changes in water flow, such as during floods or when water is concentrated in certain areas. This action can deepen and widen rivers and streams, shaping the channel over time.
The flow of water from precipitation or snowmelt that occurs on a seasonal basis, typically during specific times of the year, such as spring thaw or during the rainy season. This runoff can affect the hydrology of an area, influencing soil erosion and sediment transport.
The erosion of the base of a landform, such as a riverbank, cliff, or shoreline, caused by the action of water or ice. This can lead to collapse or the retreat of the feature, as the supporting material is removed from beneath.
A watershed that is located within or heavily influenced by urban development, where natural drainage systems are altered by buildings, roads, and other infrastructure. Urban watersheds often experience increased runoff, pollution, and reduced natural infiltration compared to natural watersheds. City planners work diligently to assess and mitigate the risks of mass movement hazards, such as mega-gullies which pose significant threats in urbanizing areas. Mega-gully formation has been linked to a combination of rainfall and failures in water resources infrastructure (WRIFs). For example, unstable soils in drainage lineswater main breaks or leaking water supply pipe caused by rainfall-induced gully erosion undermined supply lines, leading to sudden and dramatic mega-gully formation. Abrupt earth surface hazards, occurring within a matter of hours, are particularly concerning from both a safety and damage standpoint, as there is limited time for warnings or emergency responses.
