Here’s the introduction to the (as yet unpublished) report from the Wild Trout Trust from April 2021:

The Sid drains a catchment of around 3900ha over its 16km length. Rising around Burnt Common to the East of Ottery St Mary, it flows south, through the village of Sidbury, to the town of Sidmouth where it discharges into the English Channel. The Sid has three main tributaries, the Roncombe Stream, the Snod Brook and the Woolbrook.
Artificial straightening above Sidmouth has disconnected the river with its flood plain and energised the channel. Historic milling and land creation are the primary reasons for such straightening. Though milling has ceased, the densely populated town of Sidmouth has spread onto the floodplains.

And here’s a description from the Living Rivers Report from 2013/14:

The Sid is a very compact river system with rich biodiversity and relatively few landholdings. This manageable size and the Sid Vale’s long history of enthusiastic community engagement combine to create a rare landscape-scale conservation opportunity: for local people to maximise the health and freshwater wildlife of an entire Devon river catchment, from source to sea.

Living Rivers Project: the Sid Vale Association, Devon Wildlife Trust and Devon Biodiversity Records Centre

With more here:

River Sid – Wikipedia

Sid Vale Association – River Sid

Plus a map of the catchment area, which almost replicates the civil parish of Sidmouth:

River Sid catchment map

File:River Sid map.png – Wikimedia Commons