Long Island Sound (LIS) is a large estuary in the northeastern United States. It separates Connecticut to the north from Long Island, New York, to the south. This waterway stretches approximately 110 miles from the East River in the west to Block Island Sound in the east, where freshwater runoff mixes with Atlantic Ocean saltwater. The Sound’s physical dimensions and underwater topography are not uniform.
Defining the Depth: Average and Maximum Measures
The depth of Long Island Sound varies significantly across its length, making an average measurement a general reference point. Across its entire basin, the average depth is approximately 63 feet (19 meters). This relatively shallow average depth is a defining characteristic of the estuary, contrasting sharply with the deep Atlantic Ocean waters just beyond its eastern entrance.
The maximum depth plunges to considerably greater levels in specific locations. The deepest point is found in the far eastern section, near the area known as “The Race.” This narrow constriction connects the Sound to Block Island Sound and the open ocean. Here, the seafloor drops to approximately 300 to 330 feet (91 to 100 meters), creating a deep, rocky trough. The western portions of the Sound are notably shallower, with depths often measuring less than 20 feet near the New York side.
Geographical Factors Influencing Depth Variation
The variation in depth, known as bathymetry, is a direct result of the Sound’s geological history as a glacial feature. The entire basin was carved out by the massive Laurentide Ice Sheet, specifically the Late Wisconsin Glacier, which covered the region roughly 18,000 years ago. As the glacier advanced and retreated, it scoured the bedrock, creating an irregular trough that became a drowned river valley.
The deepest sections of the Sound are the result of this glacial carving, forming troughs or basins in the central and eastern regions. The eastern boundary is defined by a significant underwater feature, the Eastern Sill, which acts as a rocky barrier restricting the exchange of water with the Atlantic. Similarly, a shallower ridge known as the Mattatuck Sill exists in the western part of the Sound, contributing to the reduced depths in that area.
Implications of Depth for Tides and Navigation
The Sound’s depth profile and its connections to the ocean significantly shape its tidal dynamics. The restriction of the Eastern Sill and the relatively shallow average depth cause the tidal range to amplify as the tide moves westward. This results in a minimal tidal range of about 2.5 feet at the eastern end, increasing to approximately 7.3 feet in the westernmost harbors.
The depth and constriction at The Race create very strong tidal currents, which can reach speeds of four to five knots, rapidly funneling large volumes of water in and out of the Sound. The shallow depths in the western and nearshore areas necessitate the maintenance of specific, dredged shipping channels to accommodate commercial traffic. Deep-draft vessels primarily utilize the naturally deeper eastern regions, while navigating the strong currents requires caution, especially in areas with broken ground and rocky bottoms.