Catchments are complex ecosystems with intricate relationships between the flora, fauna and non-living components such as water quality, flow regime and habitat. The balance between these living and non-living components can be changed by human activities and led to land and water degradation. This has consequences for the economic, social and environmental.
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The key to successful management of these threats is research, education, monitoring and prompt action.
Land Degradation
 Land degradation occurs when the economic and biological productivity of land is lost. This can happen, for example, when:
  • Fertile soils wind and water erosion.
  • Land clearing of indigenous plants
  • Alien plants and animal invade an area,
  • Soils are degraded by salinity, acid pollution and/or heavy metal contamination.
  • Nutrient and Soil Run-off
  • Soil Water-logging
  • Inappropriate Fire Management
  • Climate change 
  • Drought

These water and land degradation causes significant impacts on agricultural production and land, infrastructure and natural resources affecting both urban and rural communities. In Australia, about two thirds of agricultural land is degraded. The major types of land degradation are soil erosion, soil salinity, soil acidity and soil contamination. Also mentioned nation-wide are nutrient loss and soil structure decline. Soil degradation risks are extremely widespread within large areas of our catchment due to our non-wetting soils, subsoil acidity and compaction, especially in the sandplain portions of the catchment. 
Soil Erosion
When vegetation is removed from soil surfaces and structure they become susceptible to erosion either by wind or water and frequently both these agents of erosion combine to remove surface soil. It is important to appreciate that soil erosion is a natural phenomenon and has occurred in Australia over the millennia. But since European settlement Australia State of the Environment reports have indicated that the rate of soil loss has increased by orders of magnitude, being five-fold greater where native pastures have been replaced by introduced pasture species in higher rainfall areas such as our lower catchment, and being up to 50 times greater on sloping land used for cereal cropping in upper catchment. 

Water Erosion.
Water erosion occurs in different ways that vary according to vegetation cover, soil type and structure and slope. The severity of rainfall events is a major factor in determining how much soil is transported down a slope.
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Wind Erosion
​Wind erosion caused by strong winds over bare soils. A uniform layers of particles are stripped from the soil surface by wind and transported over long distances. Again, as with water erosion, nutrients adsorbed to soil particles are also transported. Larger particles are blown along the surface becoming air borne for short distances then impacting and loosening other particles. The largest particles roll along the ground for only short distances. In our catchment these strong winds are most likely to blow between March and June, predominant direction of strong winds in the region is north-west .
Land Clearing
Land clearing represents a fundamental pressure on our environment, natural resources and assets. It immediately causes the loss and fragmentation of native vegetation but also (depending on subsequent management) leads to a variety of impacts on soils, including erosion and loss of nutrients. Approximately 44 per cent of Australian forests and woodlands have been cleared since European settlement; 39 per cent was cleared before 1972 (Evans MC, 2016). See "A Million Acres A Year" documentary to learn more about this. The 3 most heavily cleared communities (mallee with a tussock grass understorey, brigalow, and temperate tussock grasslands) together previously covered more than 170,000 square kilometres of Australia, and each has less than 20 per cent of its original extent remaining (Tulloch et al. 2015).

Fragmentation of native vegetation
​Fragmentation of remnant vegetation put more pressure on the environment following historic land clearing. This adversely affect the quality and persistence of flora and fauna, due the disruption to essential ecosystem processes such as pollination, seed dispersal, and habitat regeneration. Smaller fragments also have more edges in proportion to their total area, so opportunities may increase for weed and feral animal encroachment, changed micro-environmental conditions, ingress of fire from outside the patch and other dynamic processes, further threatening the ecology of the remnant patch. The National Connectivity Index (a nationally consistent approach to characterise fragmentation, DoE 2014a), has been developed as an instrument for monitoring and prioritising the maintenance and restoration of Australia’s heavily modified landscapes. The VAST (vegetation assets, states and transitions) assessment ‘classifies vegetation condition by degree of anthropogenic modification from a benchmark condition state’ (Lesslie et al. 2010; see Condition) and thus also provides continental-scale information, but only at a relatively coarse scale. In general, fragmentation impacts will be greatest where land clearing has been greatest, both recently (Figure 1).
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Figure 1: Hutchinson et al. 2005 & Environmental Resources Information Network, Australian Government Department of the Environment and Energy, 2011
Land clearing effect on soils
Australia's unique soils and vegetation have co-evolved across the landscape over millennia. Vegetation has adapted to the frequently nutrient-poor and sporadically wet soils, and its rooting patterns and litter-fall contribute to soil structure and fertility. Clearing, especially of the predominantly deep-rooted native vegetation has many impacts on soil, changing in hydrological cycle (water cycle), nutrients, sediments and solutes.

Our soils take decades, and in some cases centuries, to adjust to the new conditions. Therefore many soils across Australia are still equilibrating to European land use. Hydrological changes include rising watertables and surface evaporation of soil water increase the salt content of surface soils. A less widely appreciated effect of clearing is that the land surface becomes more uniform—the patchiness of the native system is lost. For example, removing mounds of litter, grass tussocks and rough surfaces leaves a relatively smooth soil surface. This almost invariably leads to more rapid run-off and erosion, less effective water infiltration, and loss of the micro-environments that are required by many species. ​When organic matter is removed there is a decrease in litter and protective surface vegetation from the surface cover making the soil more prone to erosion. Stores and cycles of nutrients adjust under the new land use, but in most cases the net loss of nutrients and leakage is greater than under natural conditions. Soil carbon typically decreases to 20–70 per cent of the pre-clearing amount (Luo et al. 2010). Restoring this very large stock of carbon and biotic processes have become a recent focus for land managers otherwise known as regenerative farming.
Invasive Weeds
A weed is any plant that requires some form of action to reduce its effect on the economy, the environment, human health and amenity,  colonizes and persists in an ecosystem in which it did not previously exist. Weeds typically produce large numbers of seeds, assisting their spread and are often excellent at surviving and reproducing in disturbed environments, though can inhabit all environments; from our towns and cities through to our oceans, deserts and alpine areas.

Invasive weeds are among the most serious threats to Australia's natural environment and primary production industries. They displace native species, contribute significantly to land degradation, and reduce farm and forest productivity. Australia spends considerable time and money each year in combating weed problems and protecting ecosystems and primary production on private and public land.
Some weeds are of particular concern and, as a result, have been listed for priority management or in legislation. Our Catchment Resources​
Weed pamphlet (2018)
​Bridal Creeper Review ​
Feral Animals
Feral animals are a significant threat to biodiversity, as well as cost to farmers profitability and mental health.
see more general info: ​environment.gov.au/biodiversity/invasive-species/feral-animals-australia
Most common feral animal in our catchment are: (click to read more) If you see any feral animal in the area please report them via https://www.feralscan.org.au/
Polluted/extensive Run-off
Runoff can come from both natural processes and human activity it occurs when there is more water than land can absorb or the surface is impervious. The excess liquid flows across the surface of the land and into nearby creeks, streams, or ponds. It mainly becomes a threat when land clearing has occurred and/or is polluted. As the water runs along a surface, it picks up litter, petroleum, chemicals, fertilizers, and other toxic substances. Nutrient and fertilizer loss have a significant impact on agricultural production and could impact heavily on Oyster Harbour and its tributaries like the Kalgan River, as it has done in the past. Runoff is an economic threat, as well as an environmental one. Agribusiness loses millions of dollars to runoff every year. In the process of erosion, runoff can carry away the fertile layer of topsoil. 
Inappropriate Fire Management
Australia is the most fire-prone continent in the world (Pyne 1992). Fire has played a critical role in the evolution of Australia’s flora and fauna for millions of years and continues to be a key driver of many of its ecosystems (Bradstock et al. 2002). Fire (both its occurrence and absence) can threaten biodiversity. A species may decline and eventually be lost from an area, if the fire regimes that occur there are adverse or ‘inappropriate’ for their biology or life history traits (Noble and Slayter 1980). No single regime is optimal for all organisms and communities, but diverse regimes, within ecological limits, are essential for maintaining biodiversity. Bushfires can also threaten people, property and industry so fire management, including proactive use of fire, is necessary to both conserve biodiversity and to reduce the negative impacts of bushfires (Burrow 2008) ​​
Secondary Salinity
Soil salinity in Australia is not a new phenomenon. Salt derived from the oceans has been deposited by rain, wind and marine ingressions (land previously submerged beneath the sea) over millions of years then leached through soils into underground aquifers and ground water until natural equilibria have been established. Secondary salinity has been brought about by vegetation clearance and the way land has been used in the past 200 years. The National Land and Resources Audit in 2001 estimated that about 2.4 million ha of land across Australia is saline with a total of 5.4 million ha deemed to be at risk. There are two types of salinity – dry land salinity and salinity caused by irrigation practices. Dryland salinity being the most prevalent in our catchment.
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Dry Land Salinity
At various locations in the landscape, geology and hydrology combine to provide a point at which water enters and flows underground. When these recharge areas are cleared of deep rooted vegetation, the balance between evapo-transpiration (the movement of water from soils, through plants, and then evaporation from the leaves into the air) and the quantity of water naturally flowing underground down the slope is disturbed. The volume of water moving through the soil profile increases and the water table containing dissolved salts rises. Frequently, soil water-logging is the precursor of salinity. Dry land salinity occurs mainly in the dryer sheep/wheat zones. Areas within the catchment that are low lying stagnant flats and gently undulating slopes and are most at risk of secondary salinity and water-logging. Though secondary salinity are not widespread, however they do have the potential to have a significant impact on agricultural production and could impact heavily on Oyster Harbour and its tributaries like the Kalgan River. ​​

Groundwater

Groundwater is one of our most important natural resources. It provides us with much of the water that we use for drinking water, household uses, crop irrigation, and many other things.But, just because it's underground doesn't mean that groundwater is safe from many of the same environmental issues that surface water faces. In fact, being underground makes it more vulnerable to environmental issues because we can't easily access it for testing, regulating, and cleaning. Groundwater also moves around a lot through varies flow systems, not only underground, but also as a source of discharge into Earth's streams, rivers, lakes, and oceans. Depth to groundwater ranges from 1 m to 8 m, depending on position and depth to basement rocks. Groundwater quality depends on the landform, rainfall and distance from coastline.
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Local groundwater flow systems
Local groundwater flow systems are those where recharge and discharge of groundwater are in close proximity to each other – usually within 1-3 km (hillside scale). This kind of aquifer has a reasonable hydraulic gradient and is very responsive to land use changes. Shortly after recharge is controlled, groundwater will drain away allowing groundwater levels to drop.

Our Catchment
40 per cent of the Oyster Harbour Catchment has local-scale groundwater flow.​ Some of these areas have not been cleared for agriculture (e.g. Porongurups). The dissected landscape north-west of the catchment and along dissected rivers has local-scale groundwater flow systems. In these areas, salinity and rising groundwater are on-site issues. Therefore, management practices outside the influence of these areas will have very little or no effect on the extent of these issues. 

Local-scale groundwater flow system with intermediate aquifers
This system extends 5-10 km and generally occur across several properties and have low gradients (i.e. undulating or flat topography) resulting in little lateral groundwater flow out of the area. Consequently lower parts may experience shallow groundwater levels and soil salinity. Groundwater levels in most flat landscapes will not decline rapidly after recharge is controlled. However, treatments in gently undulating areas may result in an initial rapid decline in groundwater levels. The rate will reduce after groundwater levels drop to a certain level. Groundwater cannot drain easily because of the low (1-2 per cent) hydraulic gradient. Consequently, levels will continue to rise for a long period until significant areas become salt-affected.

Our Catchment
The gently undulating swampy areas to the north-east and east of the catchment, the flats to the south and north-east of Porongurup and swampy landscapes along coastal areas have an intermediate groundwater flow system. Therefore, this zone has a high risk of shallow watertables. Creeklines, flats and low-lying areas are where salinity may appear. It is likely that the extent will increase until a new hydrological equilibrium is reached.  Other areas have an intermediate groundwater flow system but behave like a local-scale aquifer. These include the dissected landscape in Tertiary sediments such as tributaries of the King River, north of Albany. In these areas aquifers are large and flow beneath the landscape. However many dissected creeklines have become discharge sites and are windows to the groundwater. The short hillslopes between these creeks have attributes of the landscapes with a local-scale groundwater flow system. The dissected landscape north of Albany has fresh groundwater. The dissected areas that have a low to moderate risk of shallow watertables. Creeklines, lowlying areas and some upland seepage zones are already saline and the extent will increase until a new hydrological equilibrium is reached. Quality deteriorates near the very gently undulating landscape south of Porongurup. Groundwater in the Upper Kalgan is very saline and electrical conductivity may exceed 2000 mS/m.

Climate Change 

Climate and weather data available on Australia's Government Bureau of Meteorology (B.O.M.) and we encourage you to explore this resource and find your own answers.

Climate modelling has shown that rising temperature and falling rainfall trends are expected in the 21st century for the south west of Western Australia. On average, the dry scenario indicated that the region would experience a 14 per cent reduction in average annual rainfall by 2030 and a 0.7°C rise in temperature, relative to the baseline period.
The median scenario showed a 5 per cent reduction in rainfall and the wet scenario showed only a 2 per cent reduction for the same period. In the Perth region, the dry scenario showed a distribution in annual rainfall similar to the last decade. For the South West, the range of rainfall captured by the scenarios is consistent with previous studies and also with the observed trends over much of the region.

Past observations
The South West has been getting drier since the 1960s. May–July rainfall has reduced by around 19 per cent since 1970.
Australia’s climate in 2019
Future climate projections
Over 90 per cent of global climate models for the South West show a drying trend. Mean annual rainfall for the region is projected to decrease by an average of 25 per cent under the driest case at 2050 and 4 per cent under the wettest case.
​​https://www.water.wa.gov.au/planning-for-the-future/allocation-plans/managing-water-in-a-changing-climate/climate-trends-south-west
Drought
Drought is a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use. Drought is not simply low rainfall; if it was, much of inland Australia would be in almost perpetual drought. Because people use water in so many different ways, there is no universal definition of drought. Meteorologists monitor the extent and severity of drought in terms of rainfall deficiencies. Agriculturalists rate the impact on primary industries, hydrologists compare ground water levels, and sociologists define it by social expectations and perceptions. It is generally difficult to compare one drought to another, since each drought differs in the seasonality, location, spatial extent and duration of the associated rainfall deficiencies. Additionally, each drought is accompanied by varying temperatures and soil moisture deficits. See B.O.M.
https://youtu.be/oXG5SaZCVNU?list=PLbKuJrA7Vp7naJL31deES8QAV5E0q6U_H

Reference