Geology and Geography
Most of the headwaters flow over carboniferous coal measures overlain by glacial drift and fluvial gravels. The valleys are reasonably broad for a small river and many of the tributaries meandered through their valleys in the past. The considerable urbanisation, especially in the 19th century confined most rivers to rather narrow artificial channels bordered in places such as Bridgend with concrete flood protection walls.

The valley of the Ewenny is flatter, rising as it does on the lias limestone and meandering down to Ogmore

Environment
The industrialisation of the valley of the Ogmore, especially in the 19th century, severely damaged much of the natural environment. Coal mining in particular severely affected the main river and the Rivers Garw and Llynfi. On the Llynfi coal mining persisted longer than elsewhere with St John's colliery still open into the 1990s. The Llynfi also had iron works and brick works at Tondu contributing to the levels of pollution. In the second half of the 20th century, whilst the impact of coal diminished, new industries such as cosmetics and toiletries, paper making and sewage disposal continued to maintain the Llynfi in an almost abiotic condition. Strong enforcement action against a number of industries helped to ensure a steady improvement in quality into the 21st century.

The Garw was also impacted by coal and by sewage pollution from almost untreated sewage entering via land-treatment plots. Despite his, some tributaries such as the Nant Iechyd remained unpolluted and today provides important spawning grounds for the ever increasing numbers of salmon breeding in the Ogmore.

The River Ewenny was passed bypassed by much of the early industrialisation only to have a large lowland stretch near Coychurch canalised in order to attract the Ford Motor Company to set up business in the area.

Next to the estuary of the Ogmore is a large sewage treatment plant, the Penybont works, which treats the sewage of Bridgend and area. The discharge from this works which is by marine pipe to a point below low water mark has impacted bathing water quality in the past but the addition of UV light sterilisation to the final effluent has ensured that modern bacteriological standards are now met.

Fishery
In its industrial heyday, the Ogmore was devoid of salmonid fish throughout much of its length although brown trout survived in many of the smaller tributaries. The River Ewenny was the exception which had supported and continues to supports a Grayling fishery which is very unusual in Wales.

From the 1980 onwards, Salmon and sea-tout started returning to the main river and the River Garw in ever increasing numbers but continued to avoid the chronically polluted Llynfi. However , even on the Llynfi, native brown trout numbers increased upstream of Tondu and these were occasionally supplemented by stocked trout. In all parts of the river, trout can now be found with Salmon and sea-trout almost everywhere. In addition eels remain common with Millers Thumb, Gudgeon, Stone loach and Minnow present ubiquitously.