These ruins adjacent to Gwer were part of Sennacherib's canal system.
There is a site at Negub, near the village Quwayr, a historic river crossing.
The tunnel is located on the northern bank of the Greater Zab River opposite the modern village of Quwair at the start of a canal which followed the contour firstly to the south-west and then north to finish in the area of Nimrud. Professor Oates also suggests that the canal may have continued for a further 17 km to the confluence of the Greater Zab and its tributary the Khazir. The canal is still visible along much of its course and is best shown on the map completed by Commander Felix Jones in 1852. Felix Jones mapped the call east of Negub, although he makes it a little longer than it presently appears. Further to the east he plots the path of a 7 km long "qanat type" tunnel which seems to have received water from the River Khazir and brought it to the canal east of Negub. The Greater Zab River intervenes and has destroyed any other features. There are no known ancient references to these works extending to the Khazir River and their purpose is also hard to determine given the proximity of the Greater Zab River to the canal. Davey 1985, p 49, 51; edited for brevity
The course of the canal east of the cutting in the bluff is evident for a distance of about 2.5 km where it meets the flood plain of the Greater Zab River. The river presently flows some distance to the south of this place but the growth of shoq aloneg the northern border of the plain shows that previously the river at least partially flowed past this point. The depression through which the canal would have flowed is 8 m deep in places and the gradient is very flat. This canal system may therefore have been only of limited success because of severe silting caused by the slow rate of water flow along the section of canal east of the cutting in the bluff. Davey 1985, p 50-51
The system at Negub seems to have been part of construction by several kings.
Ashurnasirpal II (884-859 BC) records that he constructed a canal called Patti-Hegalli from the Upper Zab to irrigate the land by the Tigris. his canal is said to have cut through the mountain to its summit. ... Professor Oates has identified the canal east of the Negub Tunnel as the original system built by Ashurnasirpal II. This certainly seems probably as the reference to the cut made "through the mountain to its summit" in the Ashurnasirpal text suitably describes the cutting through the bluff north of Negub. Davey 1985, p 49, 51
Also, there is a fragmental inscription from the palace at Nimrud which records that Assyrian king Tiglath Pileser III (745-727 BC) restored the Kalhu canal system, and thus may have applied resources to this site as well.
When Layard visited the tunnel at Negub he observed an inscribed stone tablet which recorded the work of Esarhaddon (681-669 BC) who, it appears, may have been responsible for constructing the tunnel which now remains open. Unfortunately that text was destroyed soon after Layard's visit. Davey 1985, p 49
There are two tunnel systems. Both consist of a series of tunnels connecting the bottoms of a line of shafts (although some tunnels are blocked by debris).
The shorter and less-efficient one has become all clogged, while another larger system remains open and directly faces the current. The former is probably the earlier one. Also, considering the inscription found by Layard, the latter is certainly the later of the two.
The open, later system starts with three tunnels facing the fast-flowing Greater Zab River, evenly spaced and about 1m wide each. They are about 8m long and take a sinuous path to the first shaft. This is the only tunnel section to wind, and was likely due to how difficult it was to start the tunnel in a river. (Assyrians certainly had access to engineers who could build long, straight tunnels such as in Israel, whether there were many water tunnels.)
The shafts are battered (getting narrower at the bottom) and have stairways cut into their sides. Interestingly, Sennacherib's palace reliefs depict workers in the Balatai quarries carrying excavated material up stairways much like these. The largest shaft is 15m deep, and 15m x 4m at the bottom. The tunnels are generally 3m high with about 1-2m of silt on the bottom. Some have a circular arch; others are almost a flat-back. The longest tunnel is 14m long.
At the Negub systems, the Greater Zab River has tremendous force. This is dangerous because it can cause canal bank erosion, and had to be controlled. The tunnel system has a rise of about 3m from the river to the canal. It is normal for river diversion channels to slope upward at their start to dissipate water flow energy and reduce stream flow rate. There is also a sluice gate system in the first shaft. It consisted of a wall facing the water flow, a partition 3m higher than the current ground and with three openings. There is a stairway descending to it, and next to it are two vertical grooves cut into the side of the shaft, 0.3m wide and 0.2m deep. These likely were guides for a sluice gate which could be raised or lowered agains the partition. (Similar grooves for sluice gates were found at the entrance of the Erbil water tunnel, and at Bavian near the entrance of the Nineveh canal.)
Assyrian canal construction used enormous manpower and seem to have completed quickly. Building smaller, shorter tunnels connected by shafts allows a great number of simultaneous places for workers. (Then the excavations all met precisely.) This required precise coordination and planning. Aside from greater efficiency, having multiple tunnels and shafts allowed easier maintenance, especially for silt removal. (Although the nearby cut canals would also provide easy silt maintenance, so this could not have been the major criterion.)
The driving of a single tunnel is a slow operation because of the limited number of excavation work places. The system adopted at Negub would have involved the simultaneous sinking of three shafts and then the driving of each tunnel from both directions thus overcoming this limitation. This system can be considered a development from Assyrian quarry operations and in particular the canal work of Ashurnasirpal II who "cut through the mountain near its summit" near Negub. Instead of completely excavating a cut through the hillside, this later method was more efficient because it involved less excavation without greatly reducing the number of work places. The number of excavating work places remained high because the tunnels were comparatively short and a certain amount of sinking would have continued after the tunnels were commenced. Davey 1985, p 55
The Negub systems with their shafts and tunnels appear similar to a
qanat when described on paper. First of all, the dimensions are completely different. A qanat has narrow shafts and tunnels that may barely be passable by a person, while the Negub systems are so massive that some shafts seem to be open-air rooms. Also, the functions are different: a qanat gathers and transmits underground water, while the Negub tunnels simply conveyed river water.
Davey, Christopher. 1985. The Negub Tunnel, Iraq, Vol 47 (1985). http://www.jstor.org/stable/4200231