As the floodwaters recede, mountains of debris are left behind – sheets of plaster, loose clothes, mattresses and, of course, trees. Some debris I’ve seen in floods includes massive tree trunks weighing 5 tonnes of more, bobbing along like corks in the rapidly flowing waters.

The trees that line our creeks, rivers and floodplains are on the front line when major flooding occurs, and bear the brunt of the flood’s mighty forces. But while they are often victims of floods, trees are also our greatest allies.

From stabilising river banks with the strong grip of their roots to changing the course of floodwater, here’s how trees influence floods – and how floods can kill them.

How trees influence floods

The large and fine roots of trees, such as river red gums, bind and consolidate soil, stabilising river banks and reducing erosion. This reduces the amount of sediment entering waterways, and prevents waters down-stream becoming muddied and clogged with silt.

Large trees can also protect smaller plants such as shrubs by acting as a physical barrier, shielding other vegetation from the forceful momentum of floodwater. This is because the presence of trees slows the floodwaters’ speed, as their trunks, roots and branches block and deflect water, and change the direction of flow.

However, slowing floodwaters can also cause the flood front to widen, inundating areas further away from the usual river course. This is a major consideration when creek and river banks are being revegetated – we want to capture the benefits trees provide, but also ensure that if floodwaters slow down there’s no greater risk to property or life.

Another different but related role is that trees can prevent landslides or landslips. Indeed, landslides have occurred across flood-affected regions such as Illawarra and Kangaroo Valley in NSW, and continue to threaten people and homes.

On slopes, tree root systems consolidate soils and help prevent the movement of super saturated soil, which can flow like a liquid down hill. So it can be a problem when people remove trees from around their homes or along roads as a part of bushfire prevention programs, without thinking that cleared sites and roadside verges might be prone to landslides.

Sometimes a compromise might be a better management option. Rather than removing all trees on a slope or verge, leave some of those with large roots systems and plant trees that might slow fires or resist them. Also consider planting species that resprout after fires such as tree ferns, so their roots systems aren’t lost and the soil doesn’t erode.

Trees are also flood victims

Some trees won’t survive major floods as the water’s brute force undermines their root systems, bringing them down.

In other cases, the debris, including whole trees and large branches, acts as a battering ram on large trunks. Most big trees will survive this, but some will be repeatedly battered until the trunk, major branches or root systems fail.

These then become part of the debris that damages infrastructure, such as bridges and other trees downstream.

For most trees, floods are a fleeting event that lasts a few days or perhaps a couple of weeks. Many tree species cope well with this situation, but what happens to those that might be inundated for weeks or even months in the wake of floods?