The part of a textile mill water system which is the furthest upstream in the river is the weir. This is provided to increase and maintain the depth of water at the point where the water is required to enter the head goit via a sluice gate. The water may be led by way of the head goit either into a mill reservoir or directly into the pentrough* of the water wheel.
A common arrangement of weirs constructed to supply water to a textile mill consists of:
- A stone built curved wall in the form of a horizontal arch with its concave side facing downstream. The top surface of the stone forming the weir is as level as possible.
- Upstream of the weir an apron, also of stone, which slopes gently upwards toward the weir, is sometimes provided to ensure that the water approaches the weir flowing as smoothly as possible.
- On the downstream side of the weir another stone apron, which slopes gently downwards away from the weir, is provided to prevent the falling water from eroding the downstream bed of the river.
- Some weirs are provided with a series of stone steps on the downstream face of the weir to dissipate the kinetic energy of the water passing over the weir by causing it to change direction several times in its descent.
The system worked by taking a flow of water from the river, above the weir, via a sluice gate, through a goit or head race to the wheel. The supply was sometimes regulated by storing the water in a mill pond or dam before taking it to the wheel. The water was then taken back to the river through the tail race.
The wheel could be sited within or on the outside of the buildings and directly drove the machinery in the building through a system of belts and pulleys. It did not, unlike modern turbines, produce electricity.
* A Pentrough is an open channel (or sometimes a pipe) which directs the water into the water wheel in as smooth a manner as possible to ensure that the buckets are filled evenly.
These systems are still in existence in the valley and may sometimes be used for modern micro hydro systems.