The watershed of Brome Lake
A watershed is a portion of territory bounded by ridgelines, whose waters feed a common outlet: stream, lake, sea, ocean, etc. The common outlet of the Lake Brome watershed is the Yamaska River. The Lake Brome watershed covers an area of 186.7 km². There are approximately 3,375 housing units in the watershed for a population of about 6,000 people. Except for about 2,000 housing units in Lac-Brome, all residences are equipped with private septic systems.
Land use
13,3%
AGRICULTURAL
60,3%
FORESTRY
7,7%
ANTHROPOGENIC
14,4%
AQUATIC
Land use in the watershed partly determines whether rainwater infiltrates, runs off, or evaporates. The part of the water that infiltrates can remain near the surface, where it is partly used by plants for growth or simply evaporated. The other part of the infiltrated water reaches the groundwater table. In areas covered with vegetation, such as forested areas, the soil absorption rate is much higher than in places where the soil surface is bare. In urbanized areas, there is much less infiltration, increasing runoff and especially the pollutant loads that this water carries.
Water that runs off from a watershed disregards administrative boundaries. Thus, the Lake Brome watershed encompasses eight municipalities, in whole or in part. These municipalities are attached to two RCMs, namely the RCM of Brome-Missisquoi and the RCM of Memphrémagog.
Increasingly, public authorities recognize the need to manage water policies according to the characteristics of each watershed. This is known as integrated water management by watershed.
Municipalities in the watershed
Bolton-Ouest
43%
Lac-Brome
31%
Stukely-Sud
18%
Ste-Étienne de Bolton
5%
Canton de Shefford
2%
Bolton-Est
0,5%
Waterloo
0,07%
The sub-watersheds
Each watershed is subdivided into a number of elementary basins (sometimes called "sub-watersheds") corresponding to the catchment area of the tributaries flowing into the main watercourse. The Lake Brome watershed includes 8 main tributaries totaling 278 km and about thirty small tributaries flowing directly into the lake.
The mandate for the characterization and diagnosis of the sub-watersheds was given to a team of scientists from the Regroupement des Associations pour la Protection de l’Environnement des Lacs (RAPPEL). These reports provide valuable sources of information for the officials of the concerned municipalities who wish to apply the necessary corrections to the quality of the water running off in their ditches, on their roads, and in their watercourses.
Le lac Brome
Brome Lake is a large basin of 14.6 km². On the eastern side, it is bordered by the Foster, Gauvin, and Glenn mountains, from which the Coldbrook stream flows. The lake is mainly fed by 8 tributaries totaling 278 km.
20,6 km
PERIMÈTRE
94,6 M m3
VOLUME
D'EAU
6,3 m
PROFONDEUR MOYENNE
3,9 m3/sec
DÉCHARGE
MOYENNE
12,8 m
PROFONDEUR MAXIMALE
197,28 m (au dessus de la mer)
LIGNE DES
HAUTES EAUX
10,2 mois
TEMPS DE RENOUVELLEMENT
DE L'EAU
400+
RÉSIDENCES RIVERAINES
Le barrage Foster
The lake level and the water outflow at the lake's outlet are two very important parameters for the lake's water quality, recreational activities, and the potable water supply to downstream municipalities. Both parameters are regulated by a dam on the Yamaska River, near Route 215 South, called the Foster Dam.
In 2011, Conservation Lac Brome prepared a technical note on the management of the Foster Dam. After analyzing the tripartite agreement and operational data from 2005 to 2010, CLB found no indication that the management of the dam was generally non-compliant with the agreement. Instead, it was clarified that the challenge lies in managing extreme events, particularly periods of drought. In this context, sustainable development strategies are proposed: water conservation programs, creation of water reserves, use of efficient management tools.
Accumulation des sédiments
Les sédiments déposés dans le lac Brome sont formés par les dépôts de matières en suspension et l’accumulation d’autres substances organiques ou inorganiques qui sont présentes dans l’eau. Les particules minérales sont transportées par l’eau qui provient du bassin versant et les particules organiques proviennent des plantes et des poissons qui meurent et coulent au fond de l’eau.
Selon Prairies, il y avait en 1997, de 6 à 8 mètres de sédiments dans la partie la plus profonde du lac, et le taux d’accumulation des sédiments serait de 3 à 4 mm par année. Pour un lac en santé, le taux d’accumulation des sédiments est plutôt de l’ordre de ,5 à 1 mm par année. Selon lui, le lac s’autoalimente en phosphore, autant par le phénomène d’anoxie que par la décomposition organique.
Il y a une dizaine d’années, CLB a mesuré la profondeur des sédiments, leur granulométrie et leur teneur en phosphore. La carte suivante montre la profondeur des sédiments et la concentration en phosphore des échantillons.