Four rivers feature in the eastern, northern and western boundaries of the community, tributaries one into another.
In the east the modest Clemenstone Brook, and other small channels drain from wetlands at the eastern extremity of the community, including the Clemenstone Meadow SSSI.
These join the Afon Alun near Castle-upon-Alun.
The River Alun forms the northern boundary for 5 kilometres (3.1 mi) as it flows north-west through the Alun Gorge, a deeply incised meandering valley, cut by glacial meltwater after the last ice age.
It joins the River Ewenny south of Ewenny.
The River Ewenny then flows due west, and is the boundary of both the community and the county borough.
Flowing through Ogmore Moor, and between Ogmore village and Ogmore Castle, it joins the River Ogmore, which in turn is the county boundary, flowing south-west for 2.5 kilometres (1.6 mi) between the steep rough limestone grassland of Ogmore Down and the vast sand dunes of Merthyr Mawr Common, to join the Bristol Channel through the broad sands of 'The Flats'.

The largest area of standing water is  Pwll y Mêr.
This is a village pond within the St Brides Major Conservation Area.
The outflow from the pool enters a crevice in the limestone to join the Afon Dawel (Silent River), an underground stream.
Four rivers feature in the eastern, northern and western boundaries of the community, tributaries one into another. In the east the modest Clemenstone Brook, and other small channels drain from wetlands at the eastern extremity of the community, including the Clemenstone Meadow SSSI. These join the Afon Alun near Castle-upon-Alun. The River Alun forms the northern boundary for 5 kilometres (3.1 mi) as it flows north-west through the Alun Gorge, a deeply incised meandering valley, cut by glacial meltwater after the last ice age. It joins the River Ewenny south of Ewenny. The River Ewenny then flows due west, and is the boundary of both the community and the county borough. Flowing through Ogmore Moor, and between Ogmore village and Ogmore Castle, it joins the River Ogmore, which in turn is the county boundary, flowing south-west for 2.5 kilometres (1.6 mi) between the steep rough limestone grassland of Ogmore Down and the vast sand dunes of Merthyr Mawr Common, to join the Bristol Channel through the broad sands of 'The Flats'.
The largest area of standing water is  Pwll y Mêr. This is a village pond within the St Brides Major Conservation Area. The outflow from the pool enters a crevice in the limestone to join the Afon Dawel (Silent River), an underground stream.
St Brides Major (community)