Question: What river is the castle that a stone was taken from to build the L-plan tower for Barscobe Castle on?

Context paragraphs:
Affleck Castle: Affleck Castle, also known as Auchenleck Castle, is a tall L-plan tower house dating from the 15th century, 1 mi west of Monikie Parish Church, Angus, Scotland.  It is a category A listed building.

Brunstane Castle: Brunstane Castle (alternatively Brunston, Brunstain or Brunstone Castle) is a ruined L-plan tower house, dating from the 16th century, around 2 mi south-west of Penicuik, on the north bank of the North Esk, in Midlothian, Scotland.

Auldhame Castle: Auldhame Castle is a ruined L-plan tower house standing on a ridge above Seacliff beach, about 3 miles east of North Berwick in East Lothian, and less than half a mile from Tantallon Castle.  The castle was built in the 16th century, probably by Adam Otterburn of Reidhall, Lord Provost of Edinburgh.  It consists of a three-storey main block with a projecting stair-tower.  Part of a vaulted basement remains, but the upper floors are mostly gone.

Careston Castle: Careston Castle, also known as "Caraldston Castle", is an L-plan tower house dating from the 16th century, in Careston parish, Angus, Scotland.

Auchindoun Castle: Auchindoun Castle is a 15th-century L-Plan tower castle located in Auchindoun near Dufftown in Moray, Scotland.

Castle of St John: The Castle of St John is an early 16th-century L-plan tower house in the centre of Stranraer, in Dumfries and Galloway, southwest Scotland.  It was built by the Adairs of Kilhilt c.1510.  It has been used as a home, a court, a prison, and as a military garrison during the "Killing Times" of Covenanter persecution in the 1680s.  The castle was refurbished in the late 1980s and is now a museum.

Blairfindy Castle: Blairfindy Castle is an L-plan tower house, dating from the 16th century, around 6.5 mi north of Tomintoul, and west of the River Livet.  The tower was a hunting seat of the earls of Huntly.

Lochend Castle: Lochend House, also known as Restalrig Castle and Lochend Castle, is an occupied house, incorporating the remains of a 16th-century L-plan tower house, in Edinburgh, Scotland.  It is located in the Lochend area, about 1.5 mi east of Edinburgh Castle.  The house is protected as a category B listed building.

Threave Castle: Threave Castle is situated on an island in the River Dee, 2.5 km west of Castle Douglas in Dumfries and Galloway, south-west Scotland.

Barscobe Castle: Barscobe Castle is a 17th-century tower house in Balmaclellan, Kirkcudbrightshire, Scotland.  It is a typical house of a country laird, and according to a panel above the entrance, was built in 1648.  The L-plan tower was constructed using stone taken from Threave Castle.  The main block is three storeys high with the stair wing one storey higher.  The gables have a modification of crowsteps found only in Galloway.  It is a fine example of a mid-17th-century house which was unoccupied for many years until 1971 when it was restored.  It has a modern byre (barn) attached, which has been converted into a garage.  Barscobe Castle is a category A listed building.

Supporting paragraphs: Barscobe Castle Threave Castle
Answer: River Dee