A reservoir in September is a different water from the same reservoir in July, and understanding why makes the back end far less of a lottery. Most of it comes down to something that happens quietly under the surface every autumn.
Turnover, and why it matters
Through summer a deep reservoir separates into layers. Warm water sits on top, cold water sits at the bottom, and a band called the thermocline divides them. The bottom layer gradually loses oxygen because nothing is mixing it.
That is why summer trout are so often found at a specific depth. They are sitting in the band that has both a comfortable temperature and enough oxygen.
As autumn cools the surface, the top layer becomes dense enough to sink, and the whole lake mixes. That is turnover. Once it has happened, temperature and oxygen are broadly uniform from top to bottom.
What that means for you
The fish are no longer restricted to a band. They can be anywhere in the water column, which sounds harder and in practice usually is, at least for a week or two.
During active turnover the fishing often goes genuinely poor for a few days. The water can look slightly murky and the fish scatter. If you have a bad session on a reservoir in early October and everything felt dead, turnover is frequently the answer.
After it settles, the fishing usually improves markedly, and now the fish can be caught shallow in places that were unproductive all summer.
Search depths rather than assuming
The tactical response is simple. Stop assuming a depth and start searching.
I will start a back end session with a countdown line and work through counts systematically, five, ten, fifteen, twenty, until something happens. That is more efficient than committing to a floating line because it worked in August.
Once you find the depth, stay on it. Back end fish tend to be at the same depth all over the water rather than scattered through the column.
Picking a bank
Wind is the main factor and the general rule holds: fish the bank the wind is blowing onto. The wind pushes surface food, fry and warmer surface water against that shore and the fish follow it.
Two exceptions. A very cold northerly or easterly can push fish off the windward bank rather than onto it. And on a bright, flat calm day the sheltered bank with some depth close in is often better than an empty windward shore.
Beyond wind, the features that matter in autumn are the dam wall, where there is depth close to the bank, aeration towers, and any remaining weed. Fry gather around all three.
Water levels drop and reveal things
Most reservoirs are lower at the back end after a dry summer, and that is genuinely useful. Features that are underwater in spring, old walls, roads, stream beds, foundations, become visible.
Walk the exposed margins with your phone out and photograph what you see. Next spring when the water is back up you will know exactly where those features are, and features hold fish.
This is the single most valuable hour you can spend on a reservoir all year and almost nobody does it.
Fisheries wind down too
Practical point. Back end stocking policies vary enormously. Some fisheries keep stocking to the last day, some stop weeks before the close and let the water fish out.
A phone call tells you which you are dealing with. Ask when they last stocked and whether the fry feeding has started. A fishery that has not stocked for a month and has no fry activity is a hard day.
The best of it
Get the timing right and the back end gives you the best conditioned fish of the year on an empty water. Mild, overcast, breezy October afternoon, fish moving on the windward bank, nobody else about. It is worth the days that do not work.
Gear that fits this
Gear that suits what this covers, by type.
Fly lines: Scientific Anglers Amplitude Smooth MPX Weight Forward Fly Line #5, Wychwood Connect Series 2 Little Dipper 5ft Sink Tip Fly Line #7 and Airflo Superflo Power Taper Floating Fly Line WF4F Sunburst / Purple.
For leaders and tippet, Frog Hair 100% Flourocarbon 25m Spool 2X/10.0lb, Orvis Mirage Flurocarbon Tippet 2x 9.5lb 30m and Wychwood Silk Mode 50m 2.3lb are all worth a look.
Reels worth adding: Guideline Aeon Fly Reel Black/Gold #79, Shakespeare Oracle 2 Fly Reel #8/9 and Guideline NOVA Cassette Fly Reel #68 Black.
On the rods side, Shakespeare Oracle 2 EXP Fly Rod 10ft #7 6pc, Guideline Rivolt Single Handed Fly Rod #6 10' and Guideline Elevation Single Handed Fly Rod 9'9'' #6.
Sarah Marsh came to fly fishing through her father, who kept a rod on a small syndicate water in Devon. She spent her teens fishing stocked reservoirs and gradually worked her way toward wild fish on moving water – first on rain-fed freestone rivers in Wales, then on the Hampshire chalk streams she now considers home water.
She holds a level 2 APGAI fly casting qualification and guides informally on the River Test and, from June to September, on a private beat on the River Tay for Atlantic salmon. Her personal best brown trout came from the Itchen at 4lb 1oz; she’d sooner not talk about the salmon that came off at the net last August.
At The River Bend, Sarah writes the game fish species guides, fly fishing technique articles, and the legal content covering trout and salmon regulations.
Covers: Brown trout, rainbow trout, sea trout, Atlantic salmon, fly fishing, chalk streams, Scottish rivers, close season and regulation articles