Catchments, also known as watersheds and drainage basins, are fundamental units of the terrestrial landscape. Their boundaries are system boundaries for hydrological, geomorphological, and ecological processes. In brief, all precipitation that falls on a catchment drains to a single low, outlet point. This movement of water creates physical connections between all parts of a catchment. As a result, activities in one part of a catchment can influence water movement in another.

In this lab you will explore catchment attributes that control the rate and timing of surface water flow through catchments. You will differentiate hydrographs produced in response to rainfall based on catchment attributes. You will gather your own measurements of catchment characteristics using Toporama and analyze the data at two different scales.

Pre-Readings

Catchments as Systems

At the global scale, the hydrological cycle describes the movement of water between continents, oceans, and the atmosphere. Hydrologists and geomorphologists are generally more interested in assessing the hydrological cycle at the smaller scale of catchments. Catchments are a unique areal unit of the terrestrial landscape. The various names we use for this physical unit provide clues as to why catchments are unique units:

• Catchment: This area catches all water that falls onto it as precipitation.
• Watershed: This area sheds all water that falls on it to a single point.
• Drainage basin: All water that falls onto this area drains to a single point.

In summary, a catchment may be defined as an area of land that drains all water to a single outlet point. The outlet is the lowest point in the catchment. In other words, water moves from areas of higher elevation to areas of lower elevation. The boundary of a catchment is called a drainage divide, as it divides drainage into one catchment or another. It can be visualized as the highest point of land separating one catchment from adjacent catchments. Sediment eroded from a catchment by glacier ice or running water cannot be transported across the drainage divide into an adjacent catchment by water movement. Likewise, fish cannot migrate over a divide into an adjacent catchment.

Most commonly, the outlet of a catchment is a river mouth, where the river discharges water and sediment into the downstream lake, river, or ultimately, the ocean. In contrast, the outlet of some catchments is a depression in the landscape that forms a lake (at least some of the time). This type of catchment is called a closed catchment. All precipitation that falls on this type of catchment flows to the outlet where it either evaporates or infiltrates into the ground. A local example of this type of catchment is White Lake, south-east of Penticton, BC.

Drainage Networks

Each catchment contains a stream network that channels surface water to the catchment outlet. Depending on the size of the catchment, these streams range in number from one or two to tens of thousands. Each catchment has a main stream into which tributaries deposit water and sediment. The smallest tributaries, furthest from the catchment outlet, are called headwater streams. These are the streams where rivers begin. Although they carry smaller volumes of water, they often cover a significant portion of the total length of streams in a catchment, and play important roles in recharging groundwater, containing flood water, improving water quality, and providing fish and wildlife habitat.

One way of assessing the complexity of a stream network is by ordering the streams in a catchment into a hierarchy. The most common method used to define stream order is the Strahler method (Figure 19.1):

1. Starting at the uppermost points in the catchment, all headwater, or unbranched tributary streams are given a stream order of 1, and called first-order streams.
2. When two first-order streams join, the stream order increases to 2 (second-order).
3. When two second-order streams join, the stream order increases to 3, and so on.

Note that the stream order does not increase if two streams of different orders join. The highest stream order becomes the catchment order, so for example, Figure 19.1 depicts a fourth-order catchment.

The stream or drainage network can also be described on the basis of its shape, or drainage pattern. Different patterns are usually the result of structural or lithologic controls and include:

1. Dendritic: develops in regions where there are no clear-cut structural or lithologic controls, i.e., areas with homogeneous geology. The most common drainage pattern and the most energy efficient because the length of tributaries is minimized.
2. Trellis: develops in areas of strong structural or lithologic control, in particular where there are differentially eroded rock bodies and the streams flow along the strike of the less competent rocks that have been eroded. Tributaries flow down slopes to enter the main streams at right angles.
3. Parallel: often develops on steep slopes. Tributaries are parallel to each other.
4. Rectangular: develops in a faulted and jointed landscape, which directs stream courses in patterns of right angle turns.
5. Deranged: develops where the original stream network has been disturbed, has no clear geometry, and no true stream valley. Particularly common on the Canadian Shield which was deeply scoured by ice sheets during the last glaciation.

Stream Discharge

The quantity of water flowing down a stream is called stream discharge. It is measured as a volumetric flow rate, most commonly cubic metres per second (m³/s, also known as cumecs). Discharge is commonly plotted against time; this is known as a hydrograph. Discharge is controlled by catchment size, shape and relief, drainage pattern and density, climate, land cover, in-stream controls, and the underlying geology.

Generally, as catchment size increases, discharge also increases. Major exceptions to this are 1) exotic streams that have their headwaters in temperate climates and lower reaches in arid climates, and 2) rivers that from which large quantities of water are removed by human activities. These two situations commonly occur in the same river system, such as the Columbia River system in North America.

Catchments can possess an infinite variety of shapes, reflecting the history, structure and rocks of a region. Shape plays a major role in controlling the time sequence by which water enters the main stream of a catchment, and hence the timing of flood peaks (highest point on discharge curves; Figure 19.2). Elongated catchments tend to produce smaller flood peaks than rounded or tear-shaped catchments because the arrival of rainfall runoff at the mouth is staggered over a longer period of time (Figure 19.2c). In rounded or tear-shaped catchments (Figure 19.2a, b), runoff arrives at the mouth at more or less the same time from all tributaries.

Catchment relief, the difference in elevation between the highest and lowest parts of the catchment, is also controlled to a large degree by geological structure. A catchment with greater relief will have a faster runoff and discharge response to a rainfall event.

Drainage density is defined as the length of stream channels per unit area of a catchment. Drainage density reflects land use, underlying geology, and affects infiltration and the response time between rainfall and runoff. In general, higher drainage densities (Figure 19.3b) result in larger flood events because water travels shorter distances overland (a relatively slow process) before reaching a stream and hence less time to infiltrate into the ground when compared to catchments with lower drainage densities (Figure 19.3a). Because water reaches stream channels more quickly, higher drainage densities also lead to earlier flood peaks. In terms of geomorphology, it plays a role in the development of slopes.

Streams may be either perennial (always have discharge), intermittent (only discharge when there is enough water coming from upstream or groundwater, for example, glacier melt, usually seasonal) or ephemeral (only flow in response to rainfall events). The latter two are most common in arid environments. In-stream controls such as dams separate discharge at catchment outlets from rainfall events by controlling discharge to optimize power generation or minimize flood risk in downstream areas.

Land cover can affect drainage patterns (consider irrigation ditches, for example, or channelized portions of rivers) and the conversion of rainfall to infiltration versus runoff. Urban catchments, for example, tend to be covered with less permeable cover (concrete, rather than trees or grass), and so generate flood peaks that are higher and occur more quickly than an equivalent natural catchment.

Morphometric Attributes of Catchments

Catchment morphometrics (morpho = form; metrics = measures of) provide quantitative tools for comparing the control that catchment attributes have on the timing and rate of stream discharge. We will look at two catchment and one stream network metric.

One of the most common ways to measure catchment shape is using Schumm’s Elongation Ratio (E) (Equation 19.1):

Equation 19.1

[latex]E = \dfrac{D}{L}[/latex]

where D is the diameter of a circle with the same area as the catchment, and L is the horizontal distance along the longest axis of the catchment parallel to the main stream (Schumm, 1956).

Schumm (Schumm, 1956) also developed a simple measurement of catchment relief, known as Schumm’s Relief Ratio (R) (Equation 19.2):

Equation 19.2

[latex]R = \dfrac{H}{L}[/latex]

where H is the difference between the highest and lowest elevations in the catchment, and L is as defined above for E in Equation 19.1.

Note that E and R are both unitless. Ensure that D and L, or H and L have the same units prior to calculating. Common units are metres (m) or kilometres (km).

Drainage density (D_d) is defined as the length of stream channels per unit area of a catchment (units of km / km² or km^-1) (Equation 19.3):

Equation 19.3

[latex]D_{d} = \dfrac{L (km)}{A (km^{2})}[/latex]

where L is the total length of all streams in the catchment (in kilometres) and A is the area of the catchment (in square kilometres, km²).

Groundwater Connections

When precipitation falls on a catchment, some portion of the water may infiltrate into the ground, and continue below the root zone into groundwater. The amount of precipitation that reaches the groundwater is determined by the characteristics of the local geology, plant use, and the timing, volume and intensity of precipitation events.

Areas where precipitation infiltrates into groundwater are known as recharge areas, and areas where groundwater returns to the surface, for example, into a river or a lake as a spring, are known as discharge areas. Recharge and discharge areas are sometimes very close to each other (tens of metres) and sometimes hundreds of kilometres apart. They are sometimes contained within a single surface water catchment, and sometimes are not.

The types of geology that increase the amount of groundwater flow are those that contain very permeable rocks, or lots of fissures and fractures for water to easily flow through. Limestone is one example of a very permeable type of rock. Water flows more slowly through the subsurface than in surface streams due to the extra resistance provided by the rock or soil.

In many areas, surface water flow plays a more active role in landscape formation than groundwater, and hence is the focus of this lab. However, groundwater, or its removal, does also play an active role in shaping landscapes. For example, subsidence can occur when large amounts of groundwater are removed from an aquifer; the overlying land sinks, changing catchment relief. Groundwater springs also sustain headwater lakes and streams.

Introduction to Toporama

Toporama is an online mapping tool provided by Natural Resources Canada. The maps include:

• Ground relief.
• Lakes and rivers.
• Administrative and populated areas.
• Roads and transport facilities.

You can select the layers that you wish to display. You can also choose the base layer of your map (e.g., Canada Base Map or satellite imagery). Toporama enables you to measure features, annotate, and export maps of your own creation.

Please note that while Toporama is a wonderful tool, it can be slow to load. Your patience is appreciated. If you have persistent problems, clear your cookies/cache and/or reboot your computer.

Lab Exercises

In this lab you will analyze a catchment using tools available in Toporama. You will

• Learn how to delineate a catchment and assess this delineation.
• Measure catchment and stream characteristics at two different scales.
• Calculate catchment morphometrics.
• Assess the effects of catchment attributes on discharge.

You will need access to a computer with internet connection to access Toporama and a calculator. It is estimated that this lab will take 2-3 hours to complete, depending on your familiarity with Toporama.

EX1: Catchment Characteristics and Discharge

A number of lowland catchments (i.e. catchments with no appreciable relief) with different shapes, sizes and characteristics are shown on Figure 19.4. Assume that all catchments receive a similar amount of rainfall over a similar period of time from a summer storm. The hydrographs A to H in the lower half of the figure represent the discharge response of each catchment at the numbered locations.

1. Match each catchment with the appropriate hydrograph, basing your choice on the catchment characteristics that affect the discharge: size, shape, stream network, surface land cover, underlying geology, and climate. Refer back to the introductory material as necessary.

References

Schumm, S.A. (1956). The evolution of drainage systems and slopes in bad lands at Perth, Amboy, New Jersey. Geological Society of America Bulletin, 67(5), pp. 597-646. https://doi.org/10.1130/0016-7606(1956)67[597:EODSAS]2.0.CO;2

Image Descriptions

Figure 19.4. Catchments and their hydrographs.

Figure 19.4 is composed of two portions. In the top portion of the figure are eight diagrams of catchments of different sies and shapes. Flowing streams are indicated by solid lines, subsurface drainage by red long-dashed lines, and intermittent streams by dotted lines. Each catchment has a number and circle at the catchment outlet. This is the location where discharge is to be assessed. The bottom portion of the figure contains eight boxes with graphs showing the change in discharge over time. Discharge is plotted on the y axis and time is plotted on the x axis. In the top-right corner of each large box is a small box where the matching catchment number can be written. The first two matches have been completed for you.

[Return to Figure 19.4]