Found this, maybe it helps.
The best way to describe the basic layout of Rainy Lake is the letter “L”. The vertical part of this runs north-south (The North Arm). The horizontal part runs south east (The South Arm). Both the North and South arms of the lake have many long fingers that run off the main basin of the lake to the shallower back bays. The bays have many rivers and small streams running into them which allows for excellent spawning habitat for all species. The unique feature to Rainy Lake is the North Arm and South Arm are totally different from each other. While the South Arm has dark stained water similar to the colour of tea, the North Arm has much clearer water.
South Arm: The dark water of Rainy Lake’s South Arm warms up quicker than that of the North Arm. This earlier warming moves fish to spawning areas faster. Some years depending on wind and weather, spawning in the south will start up to 10 days sooner than in the north.
North Arm: This section of the lake, being clearer water than in the south, takes longer to warm, and the fish move to the shallows a little later in the season.