Legal Definitions - arm of the sea

An arm of the sea refers to a section of a river or bay where the water level regularly rises and falls due to the ocean'stides. This tidal influence, characterized by the ebb (outward flow) and flow (inward flow) of water, can extend significantly inland, reaching as far as the tide's force can push the river's freshwater upstream. Essentially, it's the part of an inland waterway that is directly affected by the ocean's rhythmic movements.

Here are a few examples to illustrate this concept:

• Urban Riverfront: Imagine a major river, like the Hudson River in New York, flowing past Manhattan. Even several miles upstream from the Atlantic Ocean, the water level along the city's piers visibly rises and falls twice daily. Boats docking at these piers must adjust for these changes in water depth.
  • Explanation: This stretch of the Hudson River is considered an "arm of the sea" because the ocean's tide directly influences its water levels, causing it to ebb and flow, despite being a river that carries freshwater further inland.
• Coastal Estuary: Consider a wide, marshy area where a smaller river meets the ocean, forming an estuary. During high tide, saltwater pushes far up into the freshwater wetlands, creating brackish conditions that support unique plant and animal life. At low tide, the water recedes, exposing mudflats and revealing the river's natural flow.
  • Explanation: This estuarine environment qualifies as an "arm of the sea" because the regular ebb and flow of the ocean tide are the dominant hydrological features, extending the sea's influence deep into the river system and shaping its ecosystem.
• Inland Bay Inlet: Picture a long, narrow channel that connects a large, almost landlocked bay to the open ocean. While the bay itself might appear calm, the water in the connecting channel experiences strong currents that reverse direction every few hours as the tide rushes in and out, filling and emptying the bay.
  • Explanation: This narrow channel functions as an "arm of the sea" because it is the conduit through which the ocean's tidal forces directly manifest, causing the water within it to consistently ebb and flow, linking the inland bay to the sea's rhythmic movements.