The River Bulbourne is a small river in Dacorum, Hertfordshire, England. The word bourne derives from the Anglo-Saxon word for a stream.[1] It is an unnavigable tributary of the River Gade, which flows into the River Colne, which in turn is a tributary of the River Thames. The Bulbourne is an example of a chalk stream, which is a watercourse that flows from chalk-fed groundwater. Chalk streams are a very rare habitat globally, with more than 85% of all the 210 chalk streams in the world found in England.[2][3][4]
The Bulbourne flows in the Chiltern Hills, part of a system of chalk downlands throughout eastern and southern England, which formed between 84 and 100 million years ago in the Cretaceous Period when the area was a chalk-depositing marine environment.[5] The valley is at the southernmost limit of the Pleistocene glaciation ice erosion of the Chiltern scarp, giving it a smooth rounded appearance. Around Berkhamsted the valley sides rise 300 ft. It is situated on the northern rim of the larger syncline or down folding of rocks called the London Basin. The underlying geology is chalk, which outcrops in places along the east side of the valley. The subsoil is predominantly a stiff reddish clay-with-flints; in the valley itself the chalk is overlain with alluvium.[6][7]
The river runs in a south-easterly direction from between Cow Roast and Dudswell in Northchurch,[8] through Berkhamsted, Bourne End and Boxmoor to where it joins the River Gade at Two Waters in Apsley near Hemel Hempstead.[9] The current total length of the river is 7 miles (11 km); from its source to its mouth it falls 30 metres (98 ft).[10][11]
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