From the Wikipedia page 
Deans Brook is a stream which runs between Mill Hill and Edgware in the London Borough of Barnet. It is a tributary of the Silk Stream, which is a tributary of the River Brent, which is a tributary of the River Thames.
The brook rises on Mill Hill Golf Course,, and passes through a culvert under the M1 motorway to Stoneyfields Park, where it is dammed to form an ornamental lake. It then passes through suburban Edgware, where is combines with the Edgware Brook to become the Silk Stream near Edgware Hospital.
The stream largely follows its natural course, but most of the banks are restrained by wooden boards, and there is little aquatic vegetation. Kingfishers and grey wagtails are regularly seen along the brook.
The only part of the brook which is accessible to the public is in Stoneyfields Park. Deans Brook, Stoneyfields Park, allotments to the east of Edgware Hospital, and wetland south of Edgware tube station sidings together form a Site of Borough Importance for Nature Conservation, Grade II.