Changes in land use on waterways.

One of the problems that land clearing and development produced is the removal of surfaces that can readily absorb and deal with water that falls as rain, with impermeable surfaces.  These can include roads, roofs and footpaths.  Because the water cannot be absorbed it is diverted into stormwater drainage systems.

 

This means that they are subject to short sharp bursts of water flows during and just after rain events.  The water flows are very high but only for short periods of time.

Because they have high flow rates when running, they are often able to pick up sediments and pollute waterways when discharging.




If sediments are being transported in this manner, they can also begin to alter the course of waterways.  In the case below the left side of the creek will continue to erode while the right will have sediment depositing on it.

In addition, pollutants can accumulate on impermeable surfaces, which means that when they eventually are washed into stormwater with rain, they can lower water quality even further.

Land clearing

When land is cleared in Australian environments, native plants with deep and extensive root systems are usually removed.  Without roots to stabilize the soils, erosion is often the result. In addition the plants that would normally soak up the water are no longer there and so the buffering capacity of the soil is reduced if large amounts of rain fall quickly.  Erosion poses the greatest problems where urban developments are taking place in the local council area.  Not only does it affect the land but will adversely affect the waterways if nearby.  More on this in a later entry.

There are a number of types of erosion.  Those most likely to affect the local area are described below.

Sheet erosion is the uniform removal of soil in thin layers by the forces of raindrops and overland flow. It can be a very effective erosive process because it can cover large areas of sloping land and go unnoticed for quite some time. Sheet erosion can be recognized by either soil deposition at the bottom of a slope, or by the presence of light colored subsoil appearing on the surface. If left unattended, sheet erosion will gradually remove the nutrients and organic matter which are important to agriculture and eventually lead to unproductive soil.

Rill erosion is the removal of soil by concentrated water running through little streamlets, or headcuts. As soil removal continues or flow increases, rills will become wider and deeper

Classical gullies are an advanced stage of channel erosion. They are formed when channel development has progressed to the point where the gully is too wide and too deep to allow machinery or livestock to safely cross. These channels carry large amounts of water after rains and deposit eroded material at the foot of the gully. They disfigure landscape and make land unfit for growing crops.

Streambank erosion can occur in 2 ways.  It can be caused by recreational boats producing bow waves that reach the shore and cause damage. It can also be the result of altered water flows which occur when land is cleared and developed.  Look at the following entry for more information.

Soils

What is soil?

The material that we call soil is a complex mixture of eroded rock, mineral nutrients, decaying organic matter, water, air and living organisms, mostly micro organisms.  Although soil is considered a renewable resource, it is produced very slowly by the weathering of rock, deposits of sediments and the decomposition of organic matter in dead organisms.

While soil contains living organisms, it is considered part of the non-living environment (abiotic).  However, its composition will dramatically affect the types of organisms in an area.  Other abiotic factors that will affect the area’s flora and fauna include; temperature, rainfall, light availability and so on.

The best way to examine soil is from side on.  This is known as soil profiling and is usually done by digging a hole and examining the features of the walls.

These horizons collectively are known as a soil profile. The thickness varies with location, and under disturbed conditions: heavy agriculture, building sites or severe erosion for example, not all horizons will be present.

The uppermost is called the organic horizon or O horizon. It consists of detritus, leaf litter and other organic material lying on the surface of the soil. This layer is dark because of the decomposition that is occurring. This layer is not present in cultivated fields.

Below is the A horizon or topsoil. Usually it is darker than lower layers, loose and crumbly with varying amounts of organic matter.  The more fertile the soil, the thicker the topsoil layer.  The topsoil layer is where most of the complex organic matter is broken down into simpler substances.  Some of these will leach downwards into the B layer.  In cultivated fields the ploughed layer is topsoil. This is generally the most productive layer of the soil. This is the layer that soil conservation efforts are focused.

As water moves down through the topsoil, many soluble minerals and nutrients dissolve. The dissolved materials leach downward into lower horizons.

The next layer is the B horizon or subsoil. Subsoils are usually lighter in colour, dense and low in organic matter. Most of the materials leached from the A horizon stops in this zone.  As a result this layer contains a significant amount of dissolved ions. 

Still deeper is the C horizon. It is a transition area between soil and parent material. Partially disintegrated parent material and mineral particles may be found in this horizon.

At some point the C horizon will give up to the final horizon, bedrock.