Definition of "mid-stream"

So I was constrained to turn again to the mid-stream.

1800, Joseph Benson, The life of the Rev. John W. de la Flechere, page 16

We steamed easily across the first part of the Tay Bridge, and then after passing over the long spans in mid-stream we coasted smoothly down the 1 in 114 gradient, and around the sweeping curve through Esplanade Station.

1947 January and February, O. S. Nock, “"The Aberdonian" in Wartime”, in Railway Magazine, page 7

The boundary should be demarcated along the mid-stream of the course at the time of the demarcation of that branch of the Fenny River indicated as the Fenny River on Survey of India Map Sheet no 79 M/15.

2010, Brig (Retd)V P Malhotra, Security and Defence Related Treaties of India, page 244

Starting from the coast at Point Playa, the line of boundary shall run in a straight line to the River Barima at its junction with the River Mururuma, and thence along the mid-stream of the latter river to its source, and from that point to the junction of the River Haiowa with the Amakura, and thence along the mid-stream of the Amakura to its source in the Imataka Ridge,...

2013, Odeen Ishmael, The Guyana Story