The St. Clair River has three distinct reaches having a length of about 39 miles, including the St. Clair Flats delta. The fall in water level from Lake Huron to Lake St. Clair is about five feet. The upper reach, extending downstream from Lake Huron to a point about three miles below the International Blue Water Bridge, is about 800 feet wide at its narrowest point and has mid-channel depths varying from about 30 to 70 feet. Maximum velocities of the St. Clair River occur in this reach near the Blue Water Bridge. The middle reach, which extends downstream for approximately the next 27 miles, is about one-half mile wide and has channel depths varying from 27 to 50 feet. Located in this reach are Stag and Fawn Islands and a middle ground shoal opposite the City of St. Clair, Michigan.
The lower reach of the St. Clair River extends about 9 miles to Lake St. Clair. In this reach the river divides into several meandering channels as it flows across a delta area, known as the St. Clair Flats, into Lake St. Clair. The channel first splits about 1-1/2 miles below Roberts Landing, as the Chenal Ecarté leaves the main channel and flows to the east. This small channel, which carries about five percent of the total river flow, meanders through marsh and wetland, splitting into several smaller channels that empty into Lake St. Clair. Three miles below Roberts Landing, at Algonac, Michigan, the river divides into two channels around Russell Island, the North and South Channels.
The North Channel flows between Harsens Island and the Michigan mainland. Upstream of Dickinson Island the Middle Channel splits away from the North Channel. Downstream of Dickinson Island these two channels divide into smaller ones, all flowing into Lake St. Clair.
The South Channel is the main navigation channel through the St. Clair Flats area. Just downstream of Russell Island a small channel, the Chematogan Channel, splits off from the South Channel. This channel flows between Squirrel and Walpole Islands into Lake St. Clair. The South Channel continues between Harsens and Squirrel Islands until the channel is split by Bassett Island. Originally, the main branch of the South Channel flowed north of Bassett Island into Lake St. Clair. After early navigation improvements, this channel became the St. Clair Flats Canal. As part of the 27-foot Navigation Project, a channel was cut across Bassett Island, which is mainly marsh, to create a straight channel into Lake St. Clair, the St. Clair Cutoff Channel. Seaway Island was created by fill and as a result of dredging for the St. Clair Cutoff. The construction of the St. Clair Cutoff Channel caused a major change in the distribution of flow among the several channels in the St. Clair Flats.
In its natural state, the St. Clair River had depths of 20 feet or more throughout most of its length, excluding isolated shoals. Near its mouth, the river divides into several winding channels having natural depths of only four to six feet. In this particular area dredging operations by private interests and by Canadian and U.S. Government agencies have caused regimen changes in the channels.
Improvements in the South Channel of the St. Clair River, including construction of the St. Clair Flats Canal, began in 1855. The opening of the East and West Channels through the Flats, in 1906, probably had some effect on levels. Since this development spanned many years and the gauge records were poor, these effects were impossible to detect. From the beginning of the present century until 1930, a minimum depth of 20 feet was generally available along the entire river.
On August 4, 1900, the Steamer Fontana was wrecked in the narrows at the head of the St. Clair River and on September 22 of that same year the Steamer Martin was wrecked near the same point. Only the superstructures and machinery of these vessels were removed. Their hulls still lie on the river bottom near the west shore, buried in sand. These wrecks have decreased the cross sectional area of the river at its narrowest point, above the Grand Trunk Railroad gauge, causing a reduction in the capacity of the river; this in turn has affected the level of Lake Huron.
In 1908, commercial interests began to remove sand and gravel from the bed of the river, increasing its discharge capacity. It was estimated that between that date and 1925 three and one-half million cubic yards of sand and gravel were removed, most of it above the Dry Dock gauge. Sand and gravel dredging was eventually prohibited in U.S. waters above Marysville. This occurred in 1925 and in Canadian waters shortly thereafter. During the period 1920-1922, dredging was performed to improve navigation. This work generally involved the removal of isolated shoals along the river. Below Algonac, commercial interests removed large quantities of sand and gravel from the North Channel. In 1925, sand and gravel dredging was prohibited in United States waters, and shortly thereafter in Canadian waters.
Two major improvements have been made on the St. Clair River since 1933, namely, dredging for a 25-foot and a 27-foot navigation project. The 25-foot project began in June 1933 and was completed in October 1936. No compensation, or replacement of the dredged material, was provided in connection with the 25-foot project, except to dump spoil material from the dredging operation into the deeper sections of the river. The 27-foot project involved significant excavation in conjunction with the dredging of a new cut-off channel, which bypassed the southeast bend in the lower South Channel. Spoil material from this project was used to create a large island between the southeast bend and the cut-off channel. Compensation works were authorized as part of the 27-foot project, but were never constructed. Due to this deepening, the river channel was more efficient and required less slope to flow the same amount of water from Lakes Michigan-Huron to Lake St. Clair. Estimates of the impacts of the 25 and 27 foot projects have been well documented.
Ice floes from Lake Huron enter the St. Clair River generally under the influence of northerly winds. An analysis of ice retardation for the period 1900-1986 indicates that less ice retardation occurred in the mid 1930s following the completion of the 25-foot channel. Construction of the new St. Clair cut-off channel (1960-1962) and further deepening of the channel to 27 feet also decreased the degree of ice retardation. This evidence indicates that the channel deepening resulted in more efficient channels for ice passage. Significant ice retardation events have occurred (April 1984) where record ice jams have reduced normal river flow by as much as 65%.