АҚШ-Канада шекарасындағы Сент-Круа өзені

River forming part of the US–Canada border

The St. Croix River (; Maliseet Passamaquoddy: Skutik) is a river in northeastern North America, 71 mile (114 km) in length, that forms part of the Canada–United States border between Maine (U. S.) and New Brunswick (Canada). The river rises in the Chiputneticook Lakes and flows south and southeast, between Calais and St. Stephen. It discharges into Passamaquoddy Bay, in the Bay of Fundy.

The river forms from the Chiputneticook Lakes (North Lake, East Grand Lake, Mud Lake, and Spednic Lake) along the Canadian–U. S. border. U. S. Geological Survey topographic maps show the St. Croix River as beginning at the 1.0 mile (2 km) outlet stream from East Grand Lake, then flowing through the short Mud Lake and entering Spednic Lake, extending 20 mile (32 km) to its outlet at Vanceboro, Maine, and the start of the river proper. Adding the section of river and lake from the outlet of East Grand Lake gives a total length of 95 mile (153 km) to the St. Croix.

The boundary issues of the St. Croix River came out of the Treaty of Paris that was signed in 1783. The geography of the river was not charted clearly until the Jay Treaty (1794) provided provisions for surveying the boundary. The boundary between Maine and New Brunswick north of the headwaters of the Saint Croix took another four decades to establish. Following the War of 1812 there was a push to settle this disputed territory north of the St. Croix on the St. John watershed and it remained in dispute until 1842. During this conflict Maine, Massachusetts and New Brunswick continued to issue some lumbering permits to the disputed territory. With or without a permit, lumbermen were in a race to cut the best timber from the land while it was under dispute. Although it was illegal to cut trees with no permit, the Saint John River enabled this activity because it increased business at the local mills, shipyards and timber ponds in New Brunswick so the officials were slow to halt the ongoing illegal activity. During this time, the lumbermen were very serious and competitive about the trees. Whoever got to the best trees first claimed them. Dynamite was used as a tool of sabotage to blow up some of the log booms that were strung across the river to catch the recently cut trees. It is also known that at times the timber men purposely sorted their logs incorrectly to attempt to confuse local officials charged with regulating timber trade and transportation. Prior to 2001, it was possible for boaters to use campsites on both sides of the river. However, boaters must now stay on the same shore that they entered from, and report their trip to the appropriate border agency.

The United States Geological Survey maintains two river flow gauges in the St. Croix River watershed. Vanceboro, Maine where the rivershed is 413 square miles (1,070 km2), 400 feet (122 m) downstream from the Spednik Lake Dam. Baring Plantation, Maine where the rivershed is 1374 square miles (3,559 km2), 5.6 mile (9 km) downstream of the nearest dam. The maximum recorded flow here is per second and the minimum per second. USGS also maintains a water chemistry monitor at Milltown, Maine where the rivershed is 1455 square miles (3,768 km2). For water year 2001, the pH ranged from 6.6 to 7.2.

HMCS St. Croix (I81), previously USS McCook (DD 252), became a Canadian ship in 1940 as part of the Destroyers for Bases Agreement. It was renamed after the St. Croix River to follow the Canadian tradition of naming destroyers after Canadian rivers while recognizing the shared national history of the ship.

New Brunswick Power has indicated their desire to remove the 138 year old Milltown Dam on the St Croix River between St. Stephen, New Brunswick (Canada), and Calais, Maine (USA). Currently, New Brunswick Power stakeholders, government officials, the Peskotomuhkati Tribe, locals, and the public are examining this proposed dam removal. Milltown Dam removal would restore Salmon Falls between St. Stephen and Calais. The Peskotomuhkati Tribe is also advocating for the removal of Milltown Dam and the full restoration of Salmon Falls. Milltown Dam was built in 1881 by New Brunswick Power and is the oldest operating hydro facility in Canada. Atlantic Salmon, shad, eels, and alewives would all benefit from this dam's removal and ongoing habitat restoration. St Croix River oxygen levels would also improve with the restoration of Salmon Falls.