The land cover monitoring along the St. Lawrence in 2015

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Indicator name: Land cover monitoring
Status: Moderate in 2015
Trend: Unchanged for the last 40 years

Highlights

Land cover in the sub-watersheds of the St. Lawrence is characterized by increases in intensive agriculture and urban sprawl. However, because the North Shore is still heavily forested, the overall status of land cover along the St. Lawrence is considered moderate.

Long description

Figure 1 has a band of colours at the top showing the five land cover statuses. The arrow indicates an overall status of moderate. The illustration shows intensive urban, road and agricultural infrastructure along one shore. The other shore has a vegetated riparian strip and very little infrastructure. The three indicators – pressure from human activities, protection of watercourses, and interconnection of natural areas – are also shown.

Problem

Complex relationships exist between the state of the St. Lawrence’s shores, land cover in its valley, and the state of the river itself. The methodological approach in this fact sheet is based on the following statements:

• Watersheds that provide good-quality water are those that are subject to little direct pressure from human activities;
• Natural areas around or along waterbodies act as buffer zones that protect ecosystems;
• Each watershed is made up of a mosaic of interconnected natural areas.

Key measures

Seven classes of land cover – open water, wetland, prairie, agricultural land, forest, bare soil, and built-up urban areas – were mapped for the seven sub-watersheds analyzed: Montreal, Yamaska River, Saint-François River, Nicolet River, the north shore of Lake Saint-Pierre, Saint-Maurice River, and Jacques-Cartier River. For Montreal, only the Canadian portion of the sub-watershed was considered. Images acquired in September 2015 by the Landsat 8 OLI¹  satellite were used for this mapping. Images circa 2000 acquired by the Landsat-5 TM and Landsat-7 ETM satellites were used for the 2000 mapping and images circa 1975 acquired by Landsat-2 MSS were used for the 1975 mapping.

1 - Courtesy of USGS : earthexplorer.usgs.gov

Long description

Figure 2 illustrates the geographic coverage of each of the subwatersheds under study in addition to the distribution of land use classes and status for each sub-basin.

Status and trends

The overall status of the three indicators, by sub-watershed, is considered good for the north shore of Lake Saint-Pierre and the Saint-Maurice River; moderate for the Jacques-Cartier, Nicolet and Saint-François rivers; moderate–poor for the Yamaska River; and poor for the Montreal area. The overall status of the seven sub-watersheds is considered moderate (Figure 2).

Pressure from human activities

The state of two sub-watersheds on the North Shore (north shore of Lake Saint-Pierre, Saint-Maurice River) was considered not of concern (in green) in 2015, with combined values ranging from 5.4% to 14.5%, based on the assessment criteria selected for this indicator. The status of the sub-watersheds on the South Shore (the Yamaska, Saint-François and Nicolet rivers), plus Montreal and the Jacques-Cartier River, was considered to require monitoring (in yellow), with values ranging from 16.9% to 58.0%.

The trend in the seven sub-watersheds is stable compared to the 2000 results, since changes in the status of these sub-watersheds are not shown for each individual year (Table 1). Data for 1975 has been included for information purposes only. For that year, all herbaceous vegetation was classified as agriculture, but in 2000 and 2015, when better-quality remote sensing images were available, natural prairie could be distinguished. That means agriculture was overestimated for 1975. This indicator does not take into account the significant degradation of water quality, due to the accumulated effect of contaminants, in the rivers along the South Shore in the final kilometres before they reach the St. Lawrence. Only the overall pressure on the sub-watersheds is evaluated.

Table 1. Pressure from human activities: Results

Agri = % of land used for agriculture in the sub-watershed.
Urb = % of urbanized land.
Status = Combined status.
Note: The year 1975 was not qualified because of the overestimation of the agricultural class. Ratings are for illustrative purposes only.

Protection of watercourses

The three sub-watersheds on the North Shore (north of Lake Saint-Pierre and the Saint-Maurice and Jacques-Cartier rivers) are not of concern, with values ranging from 79.7% to 90.3% based on the assessment criteria selected for this indicator. The sub-watersheds of the Saint-François and Nicolet rivers require monitoring, with values of 64.8% and 56.4% respectively. Lastly, the Montreal and Yamaska River sub-watersheds are of concern, with values of 40.9% and 47.2%.

Five sub-watersheds are stable in relation to the 2000 results. Only the Montreal and Yamaska River sub-watersheds show degradation due to loss of natural areas to expanded agricultural activity (Table 2). Here again, the 1975 data are included for information only, as agriculture was overestimated and natural prairie underestimated that year.

Table 2. Protection of watercourses: Results

Note: The year 1975 was not qualified because of the overestimation of the agricultural class. Ratings are for illustrative purposes only.

The high density of hydrographic networks that can exist in a sub-watershed is illustrated in Figure 3a, using the Yamaska River as an example. The enlargement on the right in Figure 3b shows the 100 m riparian buffer zone and land cover classes for some small watercourses..

Figure 3. An example of the density of the hydrographic network in the Yamaska River sub-watershed, showing the 100 m riparian buffer zones

Interconnection of natural areas

Road density plays a significant role in fragmentation of the environment, and it is also somewhat independent of, and complementary to, urban land cover. Alberti et al. (2007) and Tiner (2004) propose using road density as a measurement for assessing the degradation of water quality and of aquatic and terrestrial ecosystems.

Two sub-watersheds on the North Shore (north shore of Lake Saint-Pierre; Saint-Maurice River) are not of concern, based on the assessment criteria for this indicator. The sub-watersheds on the South Shore (Yamaska, Saint-François and Nicolet rivers) require monitoring. Lastly, two sub-watersheds (Montreal; Jacques-Cartier River) are of concern (Table 3).

Table 3. Road density, 2015

No trend has been calculated, due to the difficulty of finding information quickly about the road network in previous decades. In addition, a change in state between two periods would require a significant modification of road density over a short interval, which did not occur between 2000 and 2015.

In general, we can see that the forest class has a positive influence on the north shore of the St. Lawrence River. However, agriculture has a negative influence on the south shore and the density of the road network has a negative influence on the region of the two large municipalities – Montreal and Quebec City – as shown in Figure 4.

Figure 4. Status and trends for the three land cover indicators

Changes between 1975, 2000, 2015

The analysis of changes between two periods highlights the expansion (gain) and reduction (loss) for the various land cover classes, showing how land cover has changed over time. For this analysis, the two major urban areas, Montreal and Quebec, are considered.

Montreal region
Between 1975 and 2000 on Montreal Island and in communities to the north and south, forest cover and agricultural land was lost to urban development; the total increase in urban land cover was 225 km2 (Figure 5a). Urban expansion continued between 2000 and 2015, but in areas farther from the St. Lawrence. That expansion is almost complete on Montreal Island, and it has slowed considerably on Île Jésus (Laval) and on the South Shore of the St. Lawrence. However, there was significant urban development in the Vaudreuil area. The overall increase in urban land cover was 240 km2 (Figure 5b). What these statistics do not show is the densification of the urban fabric on Montreal Island, where some low-density urban spaces such as parking lots were replaced by high-rise apartment buildings.

Figure 5. Analysis of changes in land cover between 1975 and 2000 and between 2000 and 2015 in the Montreal area

Quebec region
The analysis of changes in land cover class for the Quebec City area shows that urban land cover increased at the expense of agriculture in the St. Lawrence Valley, by 33.7 km2, between 1975 and 2000 (Figure 6a), and by 18.6 km2, between 2000 and 2010 (Figure 6b). Loss of forest cover caused by urban expansion, especially on the Canadian Shield, increased from 13.6 km2, between 1975 and 2000, to 54.6 km2, between 2000 and 2015. Urban sprawl occurred roughly along an axis extending north and south of the Montreal area, moving farther away from the St. Lawrence.

Figure 6: Analysis of changes in land cover between 1975 and 2000 and between 2000 and 2015 in the Quebec City area

Conclusion

The overall status – moderate – can be largely explained by the following factors:

• heavy forest cover in the upper watersheds on the north and south shores of the St. Lawrence;
• the presence of buffer zones along watercourses;
• low road density, except in the Montreal and Quebec City areas.

The purpose of this exercise was to evaluate the effect of each of the seven watersheds studied on water quality in the St. Lawrence River. Although the overall status was moderate, the analysis identified some sub-watersheds as being more likely to negatively impact water quality: not surprisingly, the Montreal and Yamaska River sub-watersheds were contributing the most to water quality degradation. However, this analysis did not take into account the spatial distribution of land cover classes within the sub-watersheds.

For more information

Alberti, Marina, Derek Booth, Kristina Hill, Bekkah Coburn, Christina Avolio, Stefan Coe and Daniele Spirandelli (2007). The impact of urban patterns on aquatic ecosystems: An empirical analysis in Puget lowland sub-basins. Landscape and Urban Planning 80: 345–361.

Environment Canada and the U.S. Environmental Protection Agency (2014). State of the Great Lakes 2011. Cat No. En161-3/1-2011E-PDF. EPA 950-R-13-002. Available at https://archive.epa.gov/solec/web/pdf/sogl-2011-technical-report-en.pdf.

SOLEC (2016). Great Lakes (SOLEC) Indicators: Descriptions for Reporting in 2016. By the Ecosystem Indicators and Reporting Task Team of the GLWQA, Document in preparation, 114–120.

Tiner, Ralph W. (2004). Remotely-sensed indicators for monitoring the general condition of “natural habitat” in watersheds: An application for Delaware’s Nanticoke River watershed. Ecological Indicators 4(4): 227–243.

Prepared by

Guy Létourneau
Fresh Water Quality Monitoring and Surveillance
Environment and Climate Change Canada

Several pictograms in this document are courtesy of the Integration and Application Network site, University of Maryland Center for Environmental Science
(ian.umces.edu/symbols/).