Chapter 8 – Fluvial Geomorphology

Release Date: December 2012

Fluvial geomorphology is the study of the form and function of streams and the interaction between streams and the landscape around them. ‘Fluvial’ refers to the processes associated with running waters, ‘geo’ refers to earth and ‘morphology’ refers to channel shape. Stream morphology is dynamic and constantly changing in both space and time. A stable stream channel is in a state of equilibrium and responds physically to the streamflow and sediment it receives from upstream.

The watershed characteristics that ultimately determine the streamflow and sediment supply of a stream system are local geology, physiography and climate (Montgomery 1999). Geology and physiography act as constraints to the level of geomorphic change and determine the nature and quantity of sediment supplied to the system. Climate, specifically precipitation, provides the energy of flow for the system and directly influences its streamflow.

Figure 1: Stream morphology survey at the Forks of the Credit

Stressors modify the level of geomorphic change by affecting the balance between flow and sediment supply (Table 1). These stressors can destabilize streams, forcing them outside their natural equilibrium resulting in short-term fluctuations or long-term systematic adjustments in stream morphology.

Table 1. Examples of Stressors on Stream Morphology

Stressors

Example(s)

Climate change

Extreme weather events, droughts and floods

Land use/cover change

Clear cutting, agriculture and impervious surfaces

Hydrologic controls

Dams and impoundments

As streams respond to stressors and progress to a new state of equilibrium the stream undergoes physical change through processes of degradation, widening, aggradation and planimetric form adjustment (see Table 2 for process descriptions).

Table 2: Stream morphology Processes and Environmental Triggers

Process

Description

Triggers

Degradation

Lowering of streambed elevation through erosion and scour of bed material

  • increased flow
  • decreased sediment supply
  • increased slope due to a reduction of channel sinuosity

Widening

Increase in stream channel width through erosion

  • often follows channel degradation
  • increased flows within a degraded channel leads to erosion of both banks

Aggradation

Building-up of streambed elevation through sediment deposition

  • decreased flow
  • increased sediment supply
  • decreased slope due to irregular meander migrations

Planimetric form adjustment

Change in channel shape as seen from the air

  • straightened course through channel management activities
  • response to aggradation and widening

Understanding stream morphology can aid in management decisions. Stream morphology influences flooding patterns, erosion rates, streamflow and sediment movement and deposition. Therefore, changes in stream morphology can alter the Credit River ecosystem. For example, lowered stream depth associated with widening would impact fish communities through loss of cover, and suitable summer and winter habitat. In addition, stream aggradation leads to embedded riffle substrate and the loss of riffle habitat. Riffle habitat is important for benthic macroinvertebrate colonization, and the refuge and reproduction of particular fish species.

Stream Morphology Monitoring

Stream morphology monitoring is based, in part, on the Rapid Geomorphic Assessment (RGA). The RGA uses visual indicators to determine if the stream is stable (MOE, 2003) or undergoing physical change through aggradation, degradation, channel widening, and planimetric form adjustment. Based on these observations, stream stability status can be classified as Stable, Transitional, or In Adjustment (see Table 3 ). RGA surveys were carried out at 21 stations along the Credit River and its tributaries in autumn 2012 (Figure 2). RGA locations include 11 stations on the main Credit River and 10 stations at the lower end of Credit River tributaries. It should be noted that while the RGA is a valuable tool to assess watercourse conditions, many fluvial processes are natural and may take decades to adjust to a new equilibrium. For streams affected by urbanization, the initial destabilization period would occur quickly, followed by a series of morphological adjustments as the stream progresses to a new steady state.

Figure 2: Stream Morphology Monitoring Stations in the Credit River Watershed and their Status (2012).

Table 3: Rapid Geomorphic Assessment Classifications

Stability Status

Description

Stable

Stream morphology is within the expected range of variance for stable channels of similar type. Channels are in good condition with minor adjustments that do not impact function of the watercourse.

Transitional

Stream morphology is within the expected range of variance but with evidence of stress. Significant channel adjustments have occurred.

In-Adjustment

Stream morphology is outside of the expected range of variance for channels of similar type. Significant channel adjustments are expected to continue.

Although RGA surveys are an appropriate and widely accepted measure of stream condition, trend analyses are somewhat limited due to the qualitative nature of the data. To mitigate this, CVC also completes a full quantitative geomorphic assessment including:

  1. Detailed cross-sections (width, depth and area),
  2.  Particle size distribution (pebble counts),
  3. Longitudinal profile, and
  4. Bank composition and characteristics.

At this time the quantitative monitoring record is too short for statistical analysis. As CVC continues to quantitatively monitor stream morphology, future reports will include statistical trend analysis and establish rates of change in geomorphology. These data will provide the detail necessary to help explain aquatic organism distribution and abundance (e.g. fish and benthic invertebrates) that are used as stream health indicators.

Stream Morphology Status (2012)

The majority (71%) of the 21 monitoring stations showed some form of change in their stream morphology and were classified as Transitional (42%) or In-Adjustment (29%; Figure 3). Transitional stations are distributed throughout the entire watershed, whereas In-Adjustment stations are located predominantly in the Lower Watershed. In contrast, stable stations are located mainly in the Middle and Upper Watershed. An exception to this generalization is that Rogers Creek and Silver Creek in the Middle Watershed are classified as In-Adjustment. No stations in the Upper Watershed or along the main Credit River are classified as In-Adjustment.

Figure 3: Distribution of stations classified as Stable, Transitional or In-Adjustment in the Credit River Watershed.

In the Lower Watershed all tributary stations are In-Adjustment whereas all Credit River stations are classified as Transitional. Differing RGA classification of tributary versus Credit River stations in the Lower Watershed is likely a response to different land use in tributary catchments versus the Credit River. Tributary catchments in the Lower Watershed are located entirely within urban and/or urbanizing catchments, whereas the catchment of the Credit River encompasses all three physiographic zones which represent multiple land use types. Therefore, the effect of urbanization in the Lower Watershed on stream morphology is reduced within the main Credit River.

Stream Morphology Process

For those stations classified as Transitional or In-Adjustment (71% of stations), the dominant process was stream widening (Figure 4). It should be noted that in most cases, stream widening also occurred in conjunction with stream degradation. As the streambed lowers, the banks destabilize and become more susceptible to erosion below the rooting depth of riparian vegetation. This erosion results in bank undercutting, soil slumping and subsequently stream widening. Where both processes were present, stream widening was listed as the dominant process. There were no scenarios in 2012 where degradation occurred independently of stream widening. Therefore, degradation is not reported as a dominant process on its own. It is important to note that degradation and widening tend to be the most common processes associated with urbanization, specifically because increased impervious cover leads to more intense flow events compared to pre-development conditions (Trimble, 1997; Doyle et al., 2000).

Figure 4: Dominant process distribution among stations that exhibit a trend (Transitional or In-Adjustment)

Tributary Stations:

The most common process observed at tributary stations is stream widening, especially within the Lower Watershed where all tributaries displayed numerous indicators of widening and degradation. Abundance and severity of these indicators (e.g. erosion and bank slumping) suggests that tributaries have undergone a period of rapid destabilization, consistent with the results of similar studies in urbanizing watersheds (Doyle et al., 2000).

Credit River Stations:

Stream widening was the most common process for the Credit River, especially within the Lower Watershed and less stable reaches of the Middle Watershed. The Upper Watershed, however, was dominated by aggradational processes, especially within the vicinity of Hwy 10. Aggradation can directly affect stream biota as important riffle habitat becomes buried in sand and silt. Aggradation in these sections of the Credit River is likely the result of naturally occurring lower stream gradients and fine sediment deposition, both of which are intensified by the backwater effect from Melville dam and the regulated flow from the south dam at Island Lake.

Conclusions

Rapid Geomorphic Assessment indicated that the majority of monitoring stations (71%) showed some form of change in their stream morphology (i.e. classified as Transitional or In-Adjustment). For these stations the dominant process across the watershed was stream widening associated with increased erosion rates and sediment loading to the stream. As expected, the Credit River stations were generally more stable than tributary stations, likely because tributaries are more sensitive to local disturbance. Monitoring stations in the Lower Watershed were the most highly impacted (i.e. In-Adjustment), likely as a result of increased urbanization in this region. The Upper Watershed, however, also showed signs of change in stream morphology through aggradational processes, possibly associated with changes to streamflow by dams and impoundments. The Middle Watershed had the majority of Stable stations suggesting little change in the stream morphology of this region.

As urbanization and climate change continue to alter the Credit River ecosystem, knowledge gained through stream morphology monitoring will inform management decisions related to such things as habitat availability and erosion rates.

In Chapter 9 we will briefly leave the aquatic environment and move onto land and examine Forest Integrity in the Credit River Watershed. What is the status of Forest Integrity and what are the major challenges facing forest ecosystems in the Credit River Watershed.

Did you know?

Since 2005, the number of non-native plant species detected in the Credit River Watershed has doubled (from 8-16) with European Buckthorn (Rhamnus cathartica) and Garlic Mustard (Alliaria petiolata) being the most problematic.

 

References

Doyle, M.W., C.F. Rich, J.M. Harbor, and A. Spacie. 2000. Physical Geography 21: 155-181.

Ministry of Environment (MOE). 2003. Ontario Ministry of Environment. Stormwater Management Planning and Design Manual.

Montgomery, D.R., 1999. Process domains and the river continuum. Journal of the Ameran Water Resources Association, 35(2): 397-410.

Trimble, S.W. 1997. Science, 1442-1444.

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