When Capability Brown landscaped the grounds he filled in the canals but they can still be detected underground by LIDAR which uses a pulsed laser light. You can make out the canals and the fishing gazebo. The bricks were first robbed out and used in building the Ha-Ha. Out of interest on the right is a map of all the properties Capability Brown worked on.

(In a paper from the University of East Anglia: LANCELOT ‘CAPABILITY’ BROWN: A RESEARCH IMPACT REVIEW PREPARED FOR ENGLISH HERITAGE BY THE LANDSCAPE GROUP, UNIVERSITY OF EAST ANGLIA
http://www.capabilitybrown.org/sites/default/files/project_files2/english_heritage_research.pdf