Saltford Mill housed five waterwheels, four within the main complex of workshops and a fifth, smaller wheel, in an auxiliary workshop upstream of the main mill.  In the mill's later configuration as a brass mill, two wheels in tandem powered the rolling mill and two wheels each powered a bank of three battery hammers.  The fifth smaller wheel drove a grind stone
Of the two wheels which powered the rolling mill, the western wheel remains operational and the dilapidated remains  of the eastern wheel can be seen in-situ in its culvert.  The two wheels which powered the battery mills were both removed in the early C20th but the open leats in which they were housed  and their mounting arrangements can still be observed. 
The wheels are all undershot, this being dictated by the relatively low height of fall of the river.  Breastshot, backshot or overshot wheels are more efficient but with the volume of the River Avon available to power the mill, low efficiency was not an issue and the arrangement was quite capable of delivering the necessary torque to drive the hammers and rolls . 
The operational wheel is 18 ft 11 in (5.7 m) in diameter and is of mostly iron construction with timber being used for the starts (to which the iron floats or paddles are attached) and the alignment wedges in the wheel's hub. The dilapidated wheel appears to have been of mostly wood construction with iron being used for its axle and hub.
A Sales Catalogue of 1859 describes the mill as housing four waterwheels of 15 ft (4.6 m) diameter, which aligns with the leats that can be observed today.  However, the surviving waterwheel is larger than the wheel in that position 1859.  The larger diameter wheel appears to be a new wheel which is known to have been constructed in the 1880s to drive one of the battery mills as part of a modernisation programme.
The arrangement of wheels was altered in 1928 when the mill was adapted to house the squash court.  The 18 ft 11 in waterwheel, the largest and most modern of the wheels, appears to have been moved to its current location to drive a dynamo through a combination of chain and belt drives.  The same drive-train also powers a circular saw.  Drive shaft openings in the battery-mill wall were blocked to enable construction of the squash court and the culvert supplying water to that wheel was sealed.  The penstock gates to the other two culverts remain functional to allow the leats to be flushed to prevent silting.
The operational wheel was restored by volunteers in 2003-04, who continue to maintain the wheel and operate it routinely on open days to demonstrate to use of water power.
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Saltford Brass Mill Project
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Scheduled Monument        

Grade II* Listed Building 

Penstock Gate

The speed and power of the wheel is regulated by controlling the flow of water onto the wheel.  This is achieved by means of a Penstock Gate which comprises a moveable gate or hatch beneath a fixed breast wall.  The hatch is angled at 45 deg to the vertical so that flow through the gate is accelerated and it is the high velocity jet of water impinging on the floats or paddles that imparts power into the wheel.