The river's source is traced to a series of lakes clustered around Victoria Bridge near the boundary with Richmond County.

The river occupies a trench arching from southwest to northeast across the southern part of Cape Breton County; much of the eastern part of the river is a tidal estuary, with noticeable tides extending beyond Albert Bridge approximately 10 km (6.2 mi) upstream of the Gut.

The highly eroded bedrock is covered by a thick layer of glacial till, sands and gravels deposited during the retreat of the last ice age some 15,000 years ago.

About three kilometres from the mouth of the Mira, the river narrows and cuts a valley through bedrock to reach the Atlantic Ocean.

The carboniferous bedrock to the north of the Mira includes numerous coal seams - evidence of the area's environment 300 million years ago.

Along the southern part of its course, glacial deposits have interrupted the flow to form a chain of small lakes which are all less than 15 metres above sea level.

[7] The river's width (on some old maps and in old Sailing Directions it is referred to as a lake) and calm waters make it ideal for pleasure boating and other recreational activities; numerous cottages line its banks.

The activity has not been as prevalent in recent years due to public fears of injury and liability, and to threats made by the Cape Breton Regional Police of fines or jail time for jumpers.