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The origin of the group of navigable drains in the fens to the north of Boston lies in the Acts of Parliament passed in 1801 and 1803 to enable improvements to the drainage in the area. Navigation was a critical element of the scheme as it allowed the easy transport of agricultural goods out of the area, and in particular coal to be brought in. The engineer for the whole scheme was the noted civil engineer John Rennie, and a significant number of his works, particularly bridges, remain largely unaltered, the majority being Listed Grade II.

The principal outlets, into the tidal Haven below Boston were the Maud Foster and Hobhole Sluices. Initially all drainage was by gravity through gates, but the need to reduce water levels in the northern part of the fens led to the construction of the Lade Bank Pumping Station in 1867. A lock was provided to enable navigation to be maintained.

Mechanical pumping was progressively introduced and then modernised to increase the volume of pumping and maintain optimum water levels. One effect of this was the closure of the lock at Lade Bank. Combined with the closure of East Fen Lock, this resulted in the isolation of the southern section of the Hobhole Drain from the remainder of the network.

Today, navigation is variable, depending on water levels and the extent of clearing of weed on smaller drains in particular. Water levels are kept lower during winter than summer. This ensures maximum winter capacity during periods of higher rainfall, and equally maintains a higher water table during summer as well as providing a reservoir for irrigation. Access is from the River Witham at Anton's Gowt Lock.

The Witham Navigable Drains are well covered by the Wikipedia article Link.

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link