In my book Flyfishing North-east Victoria, I recommend anglers turn their attention to the upper reaches of many streams during the height of summer. The same general rule applies across the border in the Snowy Mountains. It’s no surprise then that as this summer has warmed up and stream flows have reduced, my friends and I have found the best action has moved upstream. But what causes this shift?

Cooler Water

Although trout in Australian streams are more tolerant of high water temperatures than some texts would have us believe, there is evidence to suggest trout will migrate into cooler tributaries to escape very warm water, or even migrate further up the main stream. (This is one of the elements being researched as part of Victoria’s wild trout program https://flystream.com/video/all-about-victorias-big-wild-trout-fishery-project/.)

With few exceptions (e.g. tailwaters) the mountain streams in south-eastern Australia are cooler the further up you go. This is due to combination of things including proximity to cold headwater springs, elevation and shade. For example, on a recent trip to Snowy Creek in the Mitta catchment, I measured the water temperature below Lightning Creek junction at 17.5 C, then 20 km downstream and an hour later, at 21 C. This is a typical pattern in summer, and it applies to tributaries as well as the main river.

Even when trout stay put, once the water temperature regularly moves into the mid-twenties (common in the mid to lower reaches of many north-east Vic and Snowy trout streams over high summer) trout become stressed and feed less. A major objective when flyfishing is not only finding trout, but finding actively feeding trout; and these are going to be more common in cooler water.

Cover

As flows reduce in mid to late summer, the amount of cover available to the fish has a lot to do with the ‘fit’ of the remaining water to the river bed. It’s a generalising again, but the lower reaches of many trout streams have over-sized channels. During winter and spring, downstream tributaries contribute substantially to a stream’s overall volume, leading to markedly higher flows than further up. However the input from these generally lower, drier parts of the catchment fade rapidly once summer arrives. Now the upper catchment provides the majority of the water and levels recede dramatically further down. In September, gauging stations on the lower reaches of most trout streams read at least double and often more than higher up; but by January the difference is often negligible. This can leave the lower reaches of a stream surrounded by lots of bare rock, gravel or sand: good for easy access and comfortable casting perhaps, but the trout are very exposed to predators.  A ‘double whammy’ is increased exposure to sunlight, with the riparian canopy now a fair way back from the water.

Meanwhile, the same stream in its upper catchment experiences a less pronounced fall in summer level, and still fills its bed from bank to bank, offering the trout physical cover from bankside vegetation and shade.

As fate would have it, as I write tropical moisture is streaming down into the Snowys and Victoria from the northern monsoon, and heavy rain is starting to fall across many of our mountain catchments. This is unequivocally great for the trout and the overall prospects for fine fishing in the weeks ahead. Temporarily at least, the high summer conditions described above will ease. Still, it’s highly likely that before autumn arrives there will be more hot, dry weather – and more reasons to head upstream.