The Ouse is a tidal estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 in northern Shapinsay

Shapinsay

Shapinsay is one of the Orkney Islands off the north coast of mainland Scotland. There is one village on the island, Balfour, from which roll-on/roll-off car ferries sail to Kirkwall on the Orkney Mainland...

, Orkney Islands

Orkney Islands

Orkney also known as the Orkney Islands , is an archipelago in northern Scotland, situated north of the coast of Caithness...

. This water body has been shown on early maps of the island in a very similar shape to its current geometry. The Ouse is fed by small rivulets and upland springs that rise on the western part of the island's northeast spur. pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 levels of these feed waters are moderately alkaline, in the range of 9.1.