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Rock Lake, Manitoba

A page about Rock Lake is included for interest because it is closely related to Pelican Lake and some are interested in both lakes.

Rock Lake is in southwest Manitoba, near Pilot Mound southeast of Brandon. about 200 kilometers southwest of Winnipeg. The north shore is in the RM of Argyle and the south shore is in the RM of Roblin. The Pembina River runs through the lake.

It is about 8 miles long and 0.75 miles wide with a surface area of about 6.6 square miles, has an average depth of 2.5 meters (8 feet) when the lake level is at 1329 feet. The deepest point is at 401.63 meters (1317.7 feet) or about 11.3 feet deep when the lake is at the normal summer target level of 1329.0 feet. It has a drainage area of about 1800 square miles. The lake is fed by four main tributaries, the Pembina River, Long River, Whitemud Creek and Badger Creek. Levels and other information will be added to this site for general interest.

 

 

A Little History

The valley was formed when melt waters from glacial Lake Souris cut into shale bedrock as they spilled eastward into Lake Agassiz. Over time tributary streams flowing into the valley have deposited sediments over the valley floor forming natural dams. These dams caused the creation of the series of shallow lakes we see today; Pelican, Lorne, Louise, Rock and Swan Lakes.

The lake has a long recreational history dating back to the early 1900's. There are about 125 cottages on the lake and a camp run by the United Church. Before 1941 levels as low as 1324 interfered with recreational use.

A stop log dam was built about three river miles downstream of the lake to regulate levels. From 1941 to about 1951 the target level was 1327 feet. Then the level was progressively raised until 1963 when it seems 1329 was settled on and that has been the objective ever since although schemes to raise the lake as high as 1337 have been considered.

In the last 20 years the Pembina, as this chart shows, has experienced very high flows and Rock Lake floods have been extraordinary. The river has made a regular habit of washing out PR 342 just east of the lake. The peak levels have been extreme but the volumes have been even more so, resulting in high water levels for extended periods throughout the valley.

 


A sand bar formed at the east end of the lake in about 1980 and this helps to limit outflows during low periods. With all the high flows of the last two decades the channel below the lake has silted up. It seems this hasn't had much impact on levels.

This chart shows how the levels have varied since 1965, each bar shows the range of levels for a given year. The lowest recorded level was 1327.15 on August 11, 1967 and the highest level was 1339.69 recorded on April 14, 2011. There have only been five dry years since 1995. From 1965 to 1995 there were only five years where the lake fell more than a foot below the 1329 summer target.

 



Rock Lake Past Levels

 

Rock Lake range

 

Rock Lake Water Levels 2018

 

Rock Lake chart

 

 

2018

As of the end of February there isn't a lot of snow around, runoff is expected to be low.

click here for some flow history

click here for Rock Lake Dam photos

Report showing estimated impact of Pelican outflows on Rock Lake