A watermill is dependent of a reliable supply of water. There is little point in building a mill until you have taken control of the river. This also explains why many mills are just outside the village to which they belong. A village wants to be near a water supply, but not TOO near because of the risk of flooding. The mill on the other hand has to be right by the river and suffers the consequences.
Sketch Map of Stour
Whitemill stands on the flood plane of the River Stour so, as you would expect, the river is quite wide at this point.

The river doglegs past the village of Sturminster Marshall, passing close by the church, before being split by an island running downstream for some distance.

The mill's builders took advantage of this island to reduce the amount of construction work they would need to do to manage the flow of the river.

They constructed two large weirs, one on each side of the island. The first weir was placed at the top of the island on the side away from the mill. This diverted the flow of the river down the eastern side of the island. The second weir was placed about halfway down on the same side as the mill. This provided a head of water which could be diverted down the leat and under the mill.
River Stour - looking West towards Sturminster Marshall from the North of the Island River Stour - Looking South down both sides of the Island
The river flows from right to left as it passes Sturminster Marshall and approaches the top of the island. The river is then split by the island. The bulk of the brick-built northern weir can just be seen on the far bank.
The ruins of the South weir South Weir - East Bastion - Fine Stone Construction
The southern weir is of much higher quality than the northern, being built of finely dressed stone. It may well be contemporary with the building of the wheel chamber. The footings in the bed of the river make a very attractive "feature" during the low water of the summer months.
The back of the Mill - viewed along the leat The exit tunnels from the wheel chamber
After being diverted by the southern weir, the river flows along the leat towards the mill. After passing the long vanished sluices the water passed under the mill to drive the wheels before finally emerging via the race tunnels.
The Tailrace
After the water has passed under the mill, it still contains considerable energy. An undershot wheel can only extract about 20% of the power in the water - if the remaining 80% were allowed to expend itself against the bridge, where the water returns to the river, the piers would soon be washed away. The mill's builders therefore placed a large tailrace pool after the mill to dissipate the energy remaining in the flow. The first arch of the bridge can be seen to the right of the picture.
This scheme was all well and good while it lasted, but in the winter of 1866 a great flood breached the island midway between the weirs causing an instant and total loss of water to the mill. The damage was so great as to be deemed beyond economic repair so the weirs fell into disrepair and all that remains today are the buttresses in the banks and the river now flows unimpeded.
Map of Leat
As part of the conservation exercise, detailed measurements were taken on the leat and tailrace to ensure that the clearance work would dig them deep enough to restore a flow under moderate river conditions.
The lawn in flood
During high summer, without the weirs, the leat dries up completely. During the winter, when the Stour floods the surrounding fields you'll find the mill with the river lapping round the doors (garden, what garden?).
Just occasionally, if you're lucky (and it is free of weeds) the tailrace will be "as smooth as a mill pond", at which times it can look like this ...
The Mill upside down