Publication (Media): Snags - a valuable but scarce resource

Publication Type:	Media Release
Publication Name:	Snags - a valuable but scarce resource

Reference Information

Whittington, John (2000) Snags - a valuable but scarce resource - Apr 27 2000, CRCFE, Canberra - Media Release.

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Snags - a valuable but scarce resource

by Dr John Whittington, Knowledge Broker, Cooperative Research Centre for Freshwater Ecology.

Trees and branches that fall into and lodge in our rivers are an essential part of the river's ecology. They provide a place for a wide range of plants and animals to live and as water flows over and around snags, they help shape the river.

Accumulations of branches and trees – snags – are a dominant feature of Australia's few undisturbed river systems. Charles Sturt's 1838 River Murray expedition described a river system full of snags "I sent a man to the Northward, who found the country intersected by deep creeks, full of decaying timber..". Since de-snagging commenced, huge numbers of snags have been removed from our rivers. Systematic de-snagging of the Murray commenced in 1858 with the launch of the snag-boat, the Grappler. Most of this de-snagging aimed to improve river navigation, because being holed by a snag was the major danger for early riverboats. The Grappler was followed by a series of other 'snag boats'. Between 1911 and the late 1960's the snag boat, the Industry, removed about three million snags from the River Murray.

Unfortunately for the river's ecology, de-snagging was highly successful. By 1973 there were only about 1,200 snags remaining in the River Murray in the 330 km's between Lock 6 and Wentworth. Removal has continued until recent times. Between 1976 and 1987 about 160 large snags were removed from each kilometre of river between Lake Hume and Yarrawonga. A major impetus of this work was the misguided belief that snag removal would increase channel capacity and therefore the rate at which water could be supplied downstream. However, despite sustained desnagging, channel capacity in this section of the Murray did not increase.

Most attempts at "improving" the river through de-snagging have resulted in detrimental effects to the river. For example, between 1960 and 1975 river-works in the Broken River in Victoria focussed on de-snagging to minimise flooding. It is now thought that this de-snagging has been a major cause of erosion of the stream bed, and because of snag removal the river now provides much less habitat for large native fish and other biota.

Fortunately de-snagging is not generally practised now, but the legacy continues.

Why are snags so good?

Snags have been described as the lowland river equivalent of marine reefs. It is the place where the greatest numbers and diversity of lowland river biota are found. Snags provide habitat for many plants and animals, from the microscopic bacteria, fungi and algae, through to large native fish and platypus. Snags that stick out of the water provide roosting places for birds and other animals such as turtles.

Bacteria, algae and fungi form a slimy coat over the snag, called a biofilm. The biofilm provides a rich food source for many invertebrates while the snag itself is a source of food for wood boring beetles and some fungi and bacteria. The biofilm also traps nutrients needed for growth from water flowing past. Snags also trap leaves and other small pieces of organic material, which then also become rich sources of food.

In Australia's lowland rivers, snags provide one of the few hard surfaces. Invertebrates like caddisfly and mayfly larvae like to attach to these hard surfaces so that they can feed from flowing water. Larger invertebrates like crayfish and shrimp hide in amongst snags. Snags that stick out of the water provides places for invertebrates like dragonflies to transform from aquatic larvae into terrestrial adults.

Snags are the most important habitat for native fish in our lowland rivers. Indeed, recreational fishers have been the greatest advocates for snags – they have long understood their value. As water flows under and around snags it erodes deep pools which are important refuges for fish such as Golden Perch. Snags also provide calm spaces, out of the fast currents, for fish to rest. Murray Cod, which can migrate hundreds of kilometres to spawn are thought to use snags as staging posts along their journey. River blackfish lay their eggs in the safety of hollow logs, while other fish lay sticky eggs that attach to the hard surfaces of snags.

Studies undertaken by the Cooperative Research Centre for Freshwater Ecology and others have shown that streams with snags are likely to have higher numbers of fish and invertebrates than those without. Attempts to re-introduce native fish to our rivers will not be successful as we would hope, unless we also provide suitable fish habitat. In many streams successful re-introduction of native fish will also probably mean re-introducing snags.

Managing snags

· Protect remaining snags
· Protect the source for future snags; and,
· Consider restoring snags through river restoration projects.

The most important action is to protect the few remaining snags in the river systems. Removing snags must be seen as an absolute last resort – for example, if they are immediately upstream and threatening an important asset such as a bridge. Even then, the snag can be removed and then replaced downstream. The best policy is to leave snags where they are. In most cases they will not cause flooding and besides, snag removal is likely to result in increased erosion and potentially greater flooding problems downstream.

Healthy, mature streamside (riparian) and floodplain vegetation is the source of most snags. To ensure a natural supply of snags into the future we have to protect and restore riparian and floodplain vegetation.

Restoring snags is a difficult and expensive task. Just how to do this successfully is being investigated by the CRC for Freshwater Ecology in the Granite Creeks area of Victoria. Australia-wide there are not enough natural snags available to fully restore all of the lowland rivers to their original state. Therefore, we will have to be smart about how and where snags are replaced. For example, how close together do snags have to be? In which part of the river do they provide the most benefit? The CRC for Freshwater Ecology is currently undertaking research to determine the answers to these and other questions about snags.

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