WATER AND VEGETATION COVER IN NETLEY-LIBAU MARSH 1990 – 2013. PHASE II REPORT: A TIME SERIES ANALYSIS BASED ON LANDSAT IMAGERY
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Date
2015-02
Authors
Watchorn, K. Elise
Journal Title
Journal ISSN
Volume Title
Publisher
Water Science and Technology Branch, Environment Canada
Abstract
Netley-Libau Marsh, the largest coastal wetland adjoining Lake Winnipeg, has been mapped by aerial
photography in the past (Grosshans et al 2004; Verbiwski 1986), indicating a trend of vegetation loss, but
a lack of historic aerial photography has limited mapping efforts to sporadic intervals. Satellite imagery,
though of a coarser spatial resolution, has the advantage of high temporal and spectral resolution. Using
Landsat images and a methodology developed in an earlier phase of this study (Watchorn 2014), a time
series of classified vegetation cover maps was produced for twelve years between 1990 and 2013. Water
cover maps were produced for another eight years within this interval, resulting in a time series
representing 20 years of this 23-year period. This time series allowed for an investigation into
relationships between the extent and distribution of Netley’s vegetation community to underlying
hydrological factors on adjoining Lake Winnipeg and the Red River, which can be used to guide future
marsh remediation measures.
The analysis of this time series indicates that the long-term trend of vegetation loss in Netley-Libau Marsh
has not been steady, nor has it been unidirectional. Observed vegetation change – both loss and gain –
was characterised by sudden dramatic changes disrupting periods of relative stasis. Lake Winnipeg water
level was identified as the major factor responsible for shifting the balance between emergent wetland
vegetation and open water. Periods of low water as short as one year had dramatic and persistent effects
on emergent vegetation cover, particularly in smaller lakes. Regenerated emergent vegetation was less
persistent in the large Netley Lake, suggesting that marsh bathymetry is dynamic. This study also
identified that Lake Winnipeg water level and Red River flow are both contributing factors which influence
the extent of wet meadows around Netley-Libau Marsh. Decreased river discharges and lake levels were
correlated with increasing use of these regions as hayed or cultivated land. Finally, the interpretation of
the cover map time series indicates the connectivity between the marsh lakes and Lake Winnipeg has
varied and is presently increasing.
Description
Keywords
netley-libeau, marsh, landsat, satellite imagery, time series, water, vegetation, hydrological