Zooming in on the site:

Zoom in on site

A bit more detail:

Go along the footpath from Abbey Meadow to the weir. The screws will be on your right, just before the entrance to the weir.  Water will flow from the top of the Abbey Stream on your left, under the footpath and into the screws, then down into the Thames. Here is a drawing superimposed on an aerial view. 

plan view with fish pass

Why not put it in the weir itself?

The big sluice gates look ideal - and technically they may be the best place - but.... the Environment Agency's top priority is flood control. If for some reason our screws had to be stopped during a flood, there would be less sluice gates remaining to carry the flood water. The EA cannot take that risk. 

Goring and Streatley is an exception. They can use the weir, because they have the long curved overfall weir that allows flood water to pass easily. Even so they have to install an extra sluice gate and they had to pay for expensive computer modelling to prove that it would be OK.

What are those blue bulges?

They are part of the fish pass, that allows fish to migrate upstream. Migrating fish swim towards a water flow, so we have to have a fish pass that discharges next to the screws - the existing one in the weir will not do. The fish come out in calm water further up the Abbey Stream, near the footbridge, so they can rest after swimming up the fish pass. The bulges are connected ponds that provide a spawning ground and a varied habitat round the edges. 

The curved structure round the entrance of the screws is mostly underwater and it is to stop the fish being sucked down again. Fish going downstream can pass through the screws without damage.