Planetary Geology on Earth: Slopes: SCAR2026 EarthCache

Planetary Geology on Earth: Slopes and Erosion (and outfalls on Earth)

This earthcache is part of SCAR2026 -  this cache should not be sought out until after 5:30 pm on May 22nd.2026

To log this Earth Cache, please answer the following EarthCache Logging Tasks (questions) from the following EC story

TASKS:

1. Drainage Patterns  Question: Are there channels, rivulets, or pathways where water concentrates during storms? How does water move down the slope?  Saskatoon's outfalls are most active during the spring melt or summer rain. Martian RSLs (streaks) appear in the warm summer. Based on the current weather today, is this slope "active" or "dormant"?

2. Measuring the Grade (Slope Steepness)  Question: Using the “10-foot rule” (two long strides), does the land drop about 1.5 feet, more, or less? Or you may choose to use the "levelling" method.  Would you describe the slope as 15%, 25%, or steeper?

3. The Outfall Exit Question: Look at where the pipe meets the ground. Did the city put riprap, concrete, or metal there? Why is it important to use these instead of leaving bare dirt under the pipe?  On Earth, rills and gullies are carved by liquid water. On Mars, scientists think some gullies are formed by "dry" processes like sliding chunks of dry ice (CO2). Look at the sediment at the "toe" (bottom) of the slope here (do not go into the outfall exit area, stay on the riverbank soil). Does the toe (or base of the riverbank) look like it was moved by a liquid (mixed together and muddy) or just fell in a dry pile?

4.Please take a photo of yourself, your geocaching dog, or a personal item such as your GPSr &c to show that you were visiting  this "South Saskatchewan River Outfall Area"  (face not required)  This photo can be inserted into the log after sending answers into the CO.

DO NOT POST ANSWERS IN YOUR LOG.   Please don’t provide the answers when logging the cache online except for the photo, for the rest of 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 so you can log your EarthCache!  You will probably find the answers you are looking for in this description page confirming what you are seeing at this outfall along the South Saskatchewan River!

The Urban Artery – Saskatoon’s Riverbank Outfalls

Welcome to this Planetary EarthCache along the South Saskatchewan River in Saskatoon. This site explores how stormwater drainage, erosion, slope stability, and urban development interact with the natural river valley landscape.

Saskatoon has more than 100 stormwater outfalls that release runoff into the South Saskatchewan River.  Although outfalls help prevent flooding in the city, they can also affect riverbank stability and water quality. Stormwater runoff may carry pollutants such as road salt, oil, fuel, litter, fertilizers, sediment, and soil into the river ecosystem.

This EarthCache focuses on the geological effects of water movement, erosion, slope processes, and drainage patterns along the riverbank.

In a natural prairie landscape, rain and snowmelt are absorbed by the soil and filtered by deep-rooted grasses. However, in an urban environment like Saskatoon, "hard surfaces"—such as roofs, sidewalks, and parking lots—prevent this natural soaking. This is why urban development increases runoff; instead of the ground acting like a sponge, water is forced to flow across the surface. To manage this, the City of Saskatoon uses an underground network of pipes that lead to over 100 outfalls along the South Saskatchewan River. An outfall is a pipe or drainage outlet that carries rainwater and snowmelt from streets, sidewalks, parking lots, and neighbourhood storm drains into the river.

The Anatomy of an Outfall

An outfall is the "mouth" of the city's drainage system. These structures are vital for preventing neighborhood flooding, but they change how water interacts with the riverbank. Instead of water trickling slowly over a wide area, an outfall creates concentrated high-energy flow.

Without proper engineering, this concentrated runoff can contribute to erosion or slumping at the base of the riverbank. The weight of the water saturates the soil, while the speed of the flow carves away the "toe" (bottom) of the hill, making the whole slope unstable.

At many outfalls, the city places large rocks called riprap, concrete, or metal structures near the pipe opening. These materials help absorb the energy of fast-moving water and prevent bare soil from washing away. If the soil beneath the pipe were left exposed, the rushing water could quickly carve deep channels into the riverbank.

The Physics of the Slope: Grade Matters

Geologists measure the steepness of the riverbank to determine its stability. This is called the The grade  or gradient (also called slope, incline, mainfall, pitch or rise.)  One way to understand slope is by comparing vertical change (rise) to horizontal distance (run). The rise is how much the ground goes up or down, while the run is how far you move across the surface. A flat area has almost no rise, while a steep hillside has a large rise over a short distance.

Image above shows the slope grades (percentages), angles in degrees, and ratios.  Image courtesy  M. W. Toews cc4.0

• 0-3% Grade: Considered "nearly level or horizontal."
• 1-8% Grade: Considered "a gentle slope."
• 8.3% Grade: Considered the maximum slope of 1:12 for an accessibility ramp.  For every 1 inch of vertical rise, you have 12 inches (1 foot) of horizontal run.
• 15% Grade: Considered "steep." If you take two long strides (about 10 feet) and the land drops 1.5 feet, you are on a 15% grade.
• 25% Grade: Very challenging; these slopes should stay undisturbed to prevent soil failure.  For every 4 meters (or feet) you walk forward horizontally, the elevation changes by 1 meter (or foot).
• 45% or more Grade: Considered "very steep" The Diagonal.
• 90% Grade or more: A cliff or overhanging rock - you are climbing up with your hands as well.
• 100% Grade: A 45-degree angle (1-foot drop for every 1-foot forward).
• Infinite % Grade: A 90-degree angle or a vertical wall.

Slope can also be described as a ratio showing how much a surface rises compared to how far it runs horizontally. It is written as “1 in X,” meaning 1 unit of vertical rise for every X units of horizontal distance.

For example, if the ground rises 1 foot for every 200 feet of horizontal distance, the slope is written as 1 in 200. This means the land is very gentle. A smaller number after “1 in” means a steeper slope, because the land rises more quickly over a shorter distance.

Measuring slope where d is distance or X and h is the change in height.

MEASURING THE HORIZONTAL (your phone or GPSr helps)

On your phone with the geocaching app open.  Open a nearby random cache straight ahead of you (i.e. if you are facing west to go up the slope, find a cache west of you, you may also choose a cache due east of you, and then climb westerly)  Open the compass screen.  Write down how many meters you are away from that cache.  Proceed to do the next step measuring vertical.

When you are at the top, look again at the compass screen, and again write down how many meters you are away from your cache used in the last step.  Subtract the lower number of meters from the higher number of meters.  Now you know how many meters you travelled horizonally (your x number)!

MEASURING THE VERTICAL the "y" number (teamwork helps)

To measure a slope, we look at how much the elevation (the "rise") changes over a specific horizontal distance (the "run"). To do this accurately, we use a technique called leveling.

While modern smartphones use GPS to track where you are on a map, they aren't very good at measuring height—vertical errors are often two to three times larger than horizontal ones. Because of this, using a sighting level is much more reliable for finding the true slope.

How Leveling Works

Leveling is an ancient method used by early civilizations to build irrigation systems, and it is still used by professional surveyors today (though they use high-tech digital tools).

The process involves "climbing" the hill in measured segments based on your eye level. As shown in the diagram below, you determine the total vertical rise by adding up the height of your instrument for every sighting you take while moving uphill.

Steps to Measure Elevation Change (the "x" number)

Regardless of the equipment you use, the basic steps remain the same:

1. Measure your "z" height: Find the exact distance from the ground to your sighting level (or your eye height).

2. Start at the bottom: Always begin at the downhill point and work your way up the slope.

3. Sight your target: Stand at your starting point and look through the level as if you are aiming an arrow. Find a spot on the hillside that aligns perfectly with your horizontal line of sight.

4. Mark the spot: This new spot becomes your next station. If you have a partner, they can stand at that spot to mark it for you. Repeat this process until you reach the top.

By counting how many times you sighted your "z" height, you can calculate the total vertical rise of the hill.  

If your phone or GPS measures in meters, it is handy to measure your height to your vision level also in meters 

When water flows down these slopes, it gains speed, forming rivulets (tiny streams) or channels that can quickly turn into deep scars in the earth.

Slope (also called grade or gradient) describes how steep a surface is compared to a flat, level line. It shows how much a piece of land rises upward as you move forward horizontally.

The steeper the slope, the more quickly water and soil move downhill. Gentle slopes allow water to spread out and soak in, while steep slopes encourage faster runoff and more erosion. Fast-moving water has more erosive power and is more likely to form:

• rills (small grooves)
• gullies (larger trenches)
• washouts
• slumping or slope failure

Image 1 above Soil at the South Saskatchewan River river bank. Image 2 Several Rills with deposition of silt at the bottom of the rills

If you look carefully at the slope between Spadina Crescent  you may see small channels, rivulets, or pathways where water repeatedly flows downhill during storms or snowmelt.

Various forms of Slumping is a type of earth movement where a large "block" of soil and rock breaks away and slides downward along a curved surface.  Do you see slumping here?

Erosion and Slope Stability

Concentrated runoff from an outfall can weaken the toe of the slope, which is the lowest part of the riverbank hill. As erosion removes support from the base of the slope, the upper soil layers may begin sliding downhill in a process called slumping.

Bare soil without vegetation is especially vulnerable to erosion. Healthy plants are important because their roots anchor soil in place and absorb water. Thick grasses, shrubs, and other vegetation help stabilize the riverbank and reduce sediment entering the river.

If vegetation is damaged or removed, runoff becomes faster and more destructive.

Comparison of Earth Slopes to slopes on Mars and the Martian Landscape

Just like on Earth, gravity is constantly at work on Mars, pulling material down hills and crater walls. While many Martian slopes look similar, they are created by very different geological "engines."

Researchers have identified seven distinct types of downslope features. To figure out which process is at work, scientists look past the basic shape of a feature and investigate three specific clues: Timing, Scale, and Terrain.

Whether you are on a riverbank in Saskatoon or the rim of a crater on Mars, gravity and water (or other fluids) follow similar rules. While the environments are millions of miles apart, the way they "scar" the landscape is remarkably similar;  its hillsides tell a story that sounds a lot like the one here in Saskatoon. By looking at the gullies and rills on the riverbank, we can better understand the mysterious "Recurring Slope Lineae" on the Red Planet.

The South Saskatchewan River slopes comparison to the mysterious slopes of the Red Planet.

1. The Starting Point: Rills and "Fingers"

On Earth (Saskatoon):

When a heavy rain hits the riverbank, water doesn't move as a solid sheet. It finds tiny weaknesses in the soil and carves small, parallel grooves called rills. These look like tiny "fingers" scratching down the hillside.

On Mars:

Mars has very similar features called Recurring Slope Lineae (RSLs). These are narrow, dark streaks that appear to grow downslope during warm seasons. Just like rills in Saskatoon, they follow the steepest path down and often appear in clusters.

2. The Growth: From Rill to Gully

On Earth (Saskatoon):

If the rain continues or an outfall pipe pours high-energy water into those rills, they begin to merge. As they get deeper and wider, they become gullies. A typical gully in Saskatoon has an "alcove" (the top bowl), a "channel" (the path), and an "apron" (the pile of dirt at the bottom).

On Mars:

Mars is covered in gullies that look almost identical to the ones at Beaver Creek, along the Qu'Appelle River and South Saskatchewan River. They have the same three parts: the alcove, the channel, and the apron. In fact, for a long time, scientists thought these must have been carved by liquid water, just like on Earth.

Gully table showing the features of a gully MGS MOC Release No. MOC2-234, 22 June 2000

3. The Differences: What Drives the Flow?

While the shapes look the same, the "fuel" behind the erosion can be very different:

4. Concentrated Flow vs. Dry Sliding

In Saskatoon, a gully often forms because of concentrated flow from an outfall or a street drain (occuring below the outfall). On Mars, some "linear gullies" are thought to be caused by blocks of dry ice (frozen CO2) sliding down sandy dunes like hovercrafts, carving a path as they go.

Even though Mars doesn't have rainstorms like Saskatchewan, the "apron" of sediment at the bottom of a Martian slope tells a geological story of sediment movement that looks hauntingly familiar to anyone standing on the banks of the South Saskatchewan.

Summary

If you see a small "rill" forming on a bald spot of the riverbank hill in Saskatoon, you are looking at a miniature version of the same physics that shapes the giant craters of the Red Planet. Both are examples of how gravity and fluids (or ice) move material from high places to low places to reach a state of slope stability.

Some of these movements are caused by liquids (like briny water), some are powered by the freezing and thawing of carbon dioxide ice, and others are simply "dry" landslides where gravity pulls down unstable rock. By comparing these clues, we can better understand if Mars is still a geologically active—or even "wet"—world today.

Pollution and Stormwater

Stormwater runoff often carries urban pollutants into the river, including:

• road salt
• oil and fuel
• litter
• fertilizers
• sediment and soil

Sometimes you may notice oily films, garbage, sand, or debris near the mouth of an outfall. Because storm drains usually flow directly into the river without treatment, reducing pollution is important for protecting river health and wildlife habitat.

People can help reduce pollution by:

• properly disposing of litter
• reducing fertilizer use
• cleaning up pet waste
• preventing oil leaks from vehicles
• keeping leaves and garbage out of storm drains

Geological Importance

This EarthCache demonstrates how water, geology, vegetation, and urban development interact along Saskatoon’s river valley. Outfalls located high on a slope may show how concentrated stormwater changes natural drainage patterns and can increase erosion and slope instability.

The South Saskatchewan River valley continues changing through:

• erosion
• runoff
• sediment movement
• slumping
• riverbank migration

This site is an important example of urban hydrology and riverbank geology in Saskatchewan.

Safety

Be careful near steep slopes, unstable banks, and wet ground. Avoid climbing on steep riverbanks or standing near eroding edges.  The levelling  method of determing slope may not be as easy with snow on the ground.