1 CHAPTER : Important Definitions
1.1 Original Condition: The Year 1787
One of the most important concepts to a book about creek rehabilitation is the concept of “Original Condition”. Original Condition relates to the state a creek was in prior to significant human alteration. Original Condition is the benchmark, the baseline, that we have in mind when we are working on any given creek rehabilitation project.
Original Condition is not always easy to define because the environment is constantly changing, not only day to day but century to century and aeon to aeon. However the most significant changes to Australia’s natural environment have occurred in the last two centuries or so since white settlement. White settlement of Australia began slowly before speeding up markedly in the late 1800’s after the discovery of gold in Victoria. With the rapid increase in human population came an increasing impact on the environment in general, and the creeks in particular.
The year 1787 has been chosen as the point before which all of Australia’s natural systems can be considered to be in Original Condition. The year 1787 is the year the “first fleet” of colonists arrived in Australia from England.
1.2 What is a Creek
Creek is a term which does not have a specific scientific definition. In fact the terms for most water ways including streams and rivers are not scientifically defined. There are some general notions, ie that creeks and streams run into rivers but as creeks and streams often run into other creeks and streams it is hard to know when type of waterway becomes another. To complicate things further the names for different types of waterway can differ between different English speaking countries.
As this book is dealing with the rehabilitation of creeks in the Australian context I have attempted to define the 3 main waterway terms, Creek Stream and River in a way that they are commonly used in Australia.
Creek
A natural waterway with a defined channel which runs intermittently
Stream
A natural waterway which runs continuously but is too small to be navigated by a rowing boat
River
A waterway which runs continuously and at times can be navigated by a rowing boat.
1.3 Other Types of Waterway
Waterway
Waterway is a good general term which broadly describes an area in the landscape through which water regularly moves.
Marsh
A slow moving or seasonally static body of water lacking a visibly incised channel. The extent of a marsh typically changes with the seasons but the boundary is most easily be identified by changes in vegetation. Vegetation in a marsh is typically more dense, lower and more lush than surrounding areas.
Currently we commonly think of a marsh as a specific area of low lying land which contains what is essentially a static body of water. However prior to 1750 and white settlement, marshes were in reality slow moving linear waterways located on land where many of our eroded creeks now run. Of course where there was sufficient grade water would have travelled at a sufficient speed to have cut through the surrounding soils to reach a rock or gravel base. However once the waterway reached flatter land and the speed of the water dropped sufficiently, then typically the waterway would slow down, drop much of its sediment, spread out and become a marsh. This marsh would then snake its way slowly towards the ocean, usually continuing on until either the gradient increased again or the waterway joined enough other waterways that the amount of water cut a channel through the marsh and it became a stream or river.
Historically the water in a marsh could be very slow moving or even static in a given season but once there was sufficient rain the water would begin moving again. Today our concept of a marsh is far more static due to the fragmentation of the landscape and the channelization of the waterways however the image of a body of water moving slowly through low dense vegetation is as important as ever if we wish to genuinely recreate sustainable landscapes.
Grassy Meadow
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1.4 Different Types of Creek
Hard Bed Creek
A hard bed creek is a creek which has a natural base of an impervious or relatively impervious material such as gravel, rock or stone. Hard bed creeks are more commonly found in hills and mountains where the velocity of water has over time enabled the creek to cut through softer over layers of soil until a permanent bottom is reached. Creeks on flatter terrain can also be hard bed creeks but in these locations particularly where the creek has become heavily channelized post 1750, however they are less common than soft bed creeks.
Soft Bed Creek
A soft bed creek is a creek with a soft or silty soil or clay bottom. In locations where the terrain is fairly flat and a creek is not suffering from significant ongoing erosion it is normal for silt from upstream to be deposited in creeks with lower velocity flows creating or reinforcing the creeks soft bed.
In many cases what are now Soft bed creeks would have been marshes prior to 1750.
In these situations the water was moving through marshes rather than along channelized creeks. Occasionally there would have been tears in the fabric of the vegetation due to flooding or uprooting of trees. These disturbances would no doubt have resulted in some localised channelization. However this channel would have been quickly revegetated and filed with sediment leaving little evidence of its prior existence.
As the clearing of land continued and stock numbers increased the marshes that had previously spread out and slowed down the water gave way to greater channelization and erosion. This disruption to the marshes continued, and continues today, to a point where the marsh turned into creek. This transformation of Marshy waterways to creeks has become so widespread that today we do not realise that many of the Soft bed creeks were once marshes and that many of the Hard bed creeks were once Soft bed creeks.
By their nature Soft bed creeks are far more vulnerable to erosion than Hard bed creeks in fact it is not uncommon for Soft bed creeks to erode to a point where, particularly in the foothills, the bed of the creek is stripped away until a rock base is reached, and they have become Hard bed creeks.
New Creek
A new creek is a term to describe a creek that prior to 1750 would have been a marsh. Ie changes to the marsh caused surface water to flow in a more restricted area leading to the creation of an incised channel or New creek.
1.5 Anatomy of a Creek
While not all Creeks contain all the following elements, all Creeks will contain some of them. One of the difficulties when discussing Creek rehabilitation is often the lack of a common vocabulary used to describe the different features found in a Creek environment. Listed below are the key physical areas in a found in most Creeks and the most widely used terms to describe them.
(Section of a creek)
(Plan of a Creek)
Low Flow Channel:
The Low Flow Channel in a creek is the point where the water, when it is flowing, flows for the majority of its time.
Toe
The Toe of the creek is the point where the water meets the side of the low flow channel. It is at this point that the friction caused by moving water is at its most constant and therefore where the majority of erosion is initiated.
High Flow Channel
Larger creeks often have a second and wider channel which occurs above the Low Flow Channel. This wider channel where water flows when the creek is experiencing high flow events is called the High Flow Channel.
Terrace
The Terrace is a flattish area that can separate the low flow channel from the high flow channel. Without this terracing it is hard to know where a low flow channel stops and a high flow channel begins
Verge
The Verge is the flat land adjacent to the top of the High flow channel. There is no clear way of defining the width of the verge but in most cases it can be considered to be up to 50m back from the top of the height flow channel. Put another way, the width of a Verge can be considered to be the same as the width of two gum trees.
Lip (sometimes also called the crest)
The edge of the Verge where it changes gradient to become the high flow channel, or if there is no high flow channel, the low flow channel.
Gully
Along the length of any creek there will be depressions in the side of the creek channel where, after high rainfall periods water joins the creek. If the water flow is frequent and abrasive enough these depressions will begin to erode enlarging the depression into gullies. Gully’s have always existed in the Australian landscapes and are the main feeder points of water to a creek. However if the natural vegetation armouring a gully floor is sufficiently damaged gullies can become a major point of erosion.
Reach
A reach is a defined length of a creek. The defined length of a creek is not determined by measurement in, for example, meters rather by creek conditions which are locally similar.
Creek Environment
Creek environment is a general term used to describe a creek and its surrounds. There are no clearly defined boundaries to the creek environment but broadly speaking it relates to all areas that are impacted by the water flows in the Creek channel. For example changes in water flow can effect the hydrology of the surrounding landscape, wildlife abundance and the types and quantities of vegetation in the surrounding landscape.
Creek*
The term Creek can most accurately be used to describe the entire area that exists between the edges of the Verges.
Possible confusion can occur as many people use the term creek to mean the actual water running in a channel.
Creek Bank*
The term creek bank is a particularly inaccurate term. It does not specify whether the speaker is relating to the side of the high flow channel, assuming there is one, the side of the low flow channel or the verge. As all the three areas “Bank” could mean are quite different use of this term without further clarification can lead to confusion.
Flood plain*
The term flood plain can most accurately be used to describe the area outside but adjacent to the high flow channel where water flows when the creek floods.
Possible confusion can occur as many people uses the term flood plain to describe flat land adjacent to a Creek created by floods that may have happened hundreds if not thousands of years ago.
Further possible confusion about what is and isn’t a Flood plain when areas that may look like they occasionally get flooded in fact don’t due to due to relatively recent Creek channelization and or flood mitigation works which may have recently occurred further up the creek.
* Terms to be used with care due to their potentially ambiguous meaning
1.6 Rehabilitation or Restoration
The terms Creek Rehabilitation and Creek Restoration are often used interchangeably however they actually have quite different meanings.
Creek Rehabilitation
A process aimed at halting the decline of a creek system and then returning it to a pre determined level of sustainability.
Creek Restoration
A process aimed at returning a creek system to its “Original Condition”. Typically this means a condition the creek system would have been in prior to 1750.
The problem with using the term Creek Restoration is that in reality it is virtually impossible to return a creek system to the condition that it was in prior to 1750.
Obviously when working with a creek that is surrounded by farm land or has become significantly channelized the ability to replicate exactly the original creek condition is not achievable. Even when a creek is located in a setting which is located in what may appear to be natural setting there will have been changes to the creek system that existed prior to 1750.
Some of the less apparent changes include things such as; a cessation of Aboriginal fire stick farming, the introduction of exotic species of animals and plants, the extinction of, or a significant variation in the number of species of native plants and animals, changed weather patens and altered hydrological conditions including altered ground water levels and flood events.
1.7 A Creek Environment in Decline
Creeks are naturally dynamic systems and therefore it is normal for the creek environment to change, some times significantly over time. More than rivers or streams, due to the fact that they regularly dry out, creeks regularly experience large fluctuations in a range of areas including water levels, species densities, rates of erosion and nutrient levels.
The difficulty is in determining what changes are part of a normal cycle and what changes can be categorised as causing permanent damage to the creek environment.
As a general rule of thumb if a Creek is undergoing significant damage as the result of a course which can’t be reversed naturally, ie it will require human intervention, then it is reasonable to consider the creek to be in decline.
Some critical components of the creek environment where permanent change can indicate a creek in decline are:
Excessove erosion
Decreasing water flow
Increasing nutrient levels
Decreasing oxygen leaves in the water
Decreasing native biodiversity
1.8 A Sustainable Creek Environment
Put simply a sustainable creek system is one which is stable for an extended period of time.
While it is possible that a single visit to a creek may be enough to determine if a creek environment is suffering permanent damage, being able to judge the stability of a creek environment is more difficult. A time frame of 10 years is considered the minimum period of time that can pass before meaningful assessment about a creeks stability can be made
As we have seen from the terms “Restoration” and “Rehabilitation” is difficult to truly return many damaged creeks in Australia to their original condition. It is however possible and in fact the first priority of any creek rehabilitation project to adjust the creeks environment so that it stabilises.