After the fire, the river holds its breath

He was in a catchment meeting last week where the nervous talk was about any heavy rain that might would wash tons of ash into the drains and creeks and find its way into the river.

There has been plenty of work done by the Catchment Management Authority already, with field workers laying straw across drainage lines and helping farmers filter water courses. But there’s a lot of ground to cover — the Longwood fire burned around 140,000 hectares, and people visiting the fire ground reported seeing layers of ash 10cm thick.

What is needed is gentle rain over a period, so grasses can shoot and hold the ash in place while it settles, rather than wash off the now blackened and barren soil.

The biggest risk to the Goulburn is from Hughes Creek, which drains a large part of the fire ground, now denuded of grasses. It was Hughes Creek that delivered the surprise punch that lifted the river to record levels in the 2022 flood, so it can carry a lot of water.

If it’s black water, it’s bad news for the fish, crays and shrimp — and organisms that keep them fed and the river healthy. The river sees blackwater events now and then — most often when heavy summer rain washes decayed leaf matter into the river — but more dramatically from fires when they burn the catchment.

The fish kills are sudden and widespread from both events and there are few tools for softening their impact.

Once a slug of black water has entered the river, there’s not much point releasing more water from upstream storages such as Eildon because it can only travel at the same speed as the black water — it will never catch up or mix.

Dilution requires clean water coming into the river in front of the slug, or as it arrives, which is tricky. But Goulburn-Murray Water and the CMA have plans in place to try — they can use parts of the irrigation channel system and streams such as the Broken River and Broken Creek to achieve some dilution of a blackwater slug.

To add to the river’s troubles, there have been unusual temperature variations below Eildon this summer, with sudden injections of warm water coming out of the storage, then returning to cold just as suddenly: the power generating company AGL sometimes stops generating during the day because solar panels have diminished demand from the grid, and it actually costs it money to feed power in. How crazy is that!

The water on top of Lake Eildon can reach as high as 27°C in summer, whereas the water through the turbines from the bottom of the lake is usually about half that. The ecology of the tail water below Eildon has developed its unique character over the past 80 years, supporting cool water species such as the platypus, which have thrived on the rich insect life.

The rapid temperature changes confuse the bugs that live on the river bed and feed the fish, shrimp and platypus as they rise through the water column on their life cycle. It confuses the wild and farmed fish, too, which rely on subtle temperature changes for their spawning triggers.

Upper Goulburn residents say the fish have gone to ground, or disappeared this year, and the insects and crustaceans the platypus live on are hard to find. Woof!