People who spend a lot of time around streams and rivers – fishing, kayaking, canoeing, or just being – know how to ‘read’ these watercourses, whether it’s to find the best fishing holes or to pick the best line to run a set of rapids. Even just to find the perfect spot where the rush of water drowns out the noise of the world around you. I’ve spent a lot of time in the mountains, studying and observing stream systems and understanding how they interact with the landscape and biota. In this post we’ll look at the differences – and similarities – between two stream systems, and the clues we observe that help us define these differences.
The images below show a stream in the Crowsnest Pass region of southwestern Alberta, and another in the Columbia Icefield area of the Canadian central Rockies.
What do these two streams have in common?
- Looking closely at both images (click to enlarge), they both have gravel substrates, and are both relatively narrow and shallow. But that may be where the visible similarities end.
How do these streams differ?
First is the difference in age – and therefore in system stability.
- The gravel substrate is more rounded (i.e., weathered) in Star Creek than at Hilda Glacier. At Hilda, the debris has either been recently exposed from beneath a glacier or deposited from surrounding slopes, so mechanical weathering processes are limited and the gravel is more angular.
- The flow in Star Creek is much more confined within a channel than flow in Hilda stream. At Star, the banks are anchored with riparian vegetation, and there’s not much evidence of stream channel change – at least in this image. At Hilda, vegetation hasn’t yet had a chance to become established – thus the stream banks are unconsolidated and the stream channel moves depending on water power and bank erodibility.
Second is the difference in seasonal flow cycles – and their impact on stream form.
In hydrology we often talk about ‘events’, which represent things like spring snowmelt or big rainfalls. These add high volumes of water to stream systems and have large impacts on water quality and stream morphology. Usually we look at the magnitude (size) and frequency (how often) of events.
Both Star and Hilda streams are subject to high magnitude events from spring runoff and spring/summer rainfalls. In Hilda we also get events from glacier melt, which usually occur later in the season (Aug/Sep)
This is what we observe in Star Creek after a high magnitude event.
Unfortunately I don’t have pictures handy, but at Hilda Glacier, large magnitude events completely reorganize the entire stream system across the proglacial area. The channels are so shallow that it doesn’t take much to overflow their banks, and the lack of vegetation means that erosion rates are high. This means the water can easily carve new channels across the landscape.
I’ve visited Hilda annually since 1998, and while the stream system stayed remarkably consistent until 2010, in 2011 the Rockies had unusually high summer rainfalls that caused debris flows up and down the Icefields Parkway. This completely reorganized the stream system at Hilda, making it entirely unfamiliar to us.
Third is the difference in external disturbances acting on each stream system.
The Crowsnest Pass is a wildfire-dominated ecosystem, where fires occur on a ~70-100 y time interval due to the dominance of lodgepole pine. Wildfire affects stream bank erosion, changes stream bed material, and alters riparian vegetation. Below are pictures from a watershed immediately adjacent to Star Creek that was burned in 2003.
At places like Hilda, the greater landscape disturbance stems from glacier retreat. The Columbia Icefield area is not as susceptible to fire, being dominated by Englemann spruce and subalpine fir with large portions of the landscape above treeline. However, as glaciers thin and retreat, both streamflow and the landscape in front of the glacier are affected.
Complex hydrological processes in recently deglaciated landscapes complicate streamflow, and contribute to its variability on both an event and a seasonal basis. Glacier retreat in broad, flat valleys exposes a debris-covered plain that hides sinkholes, melting stagnant ice, and groundwater aquifers that interact with surface streamflow. Surface streams thus flow in braided patterns, disappear and reappear, and are fed by various melting ice sources even far downvalley of the glacier itself.
So what have we observed?
- Two mountain stream systems. One older and more established, the other newer and more dynamic. Both experience change in response to hydrologic events, but the magnitude of the change is lower in the more established system. Both are subject to landscape level disturbances, but the nature of these disturbances varies and has differing effects on streamflow.
Everything we observe – those prompts that give us a ‘feel’ for the stream system – provides clues as to what type of stream system we’re looking at and how it might change over time. Next time you’re out fishing, kayaking, canoeing or just sitting relaxing, think of what your observations are telling you about the stream. What other clues do you rely on to tell you something about streams?