Soils - Tasmania

Tasmania

Soils - erosion and sediment transport

Soil erosion is a natural process-occurring more in landscapes with high rainfall intensity or steep slopes. The shallow stony soils that cover much of the coastal ranges and the steeper semi-arid lands have been naturally eroded; where the protective vegetation cover is removed or degraded by clearing, tillage or overgrazing risks of sheetwash erosion are increased and rill and gully erosion are introduced. Associated degradation of riparian vegetation has also accelerated erosion of creek and river banks. In arid and semi-arid landscapes, reduced vegetation cover also accelerates wind erosion (reported in the Australian State of Environment 2001).

Soil erosion can reduce on-site productivity through loss of fertile topsoil, and associated water-holding capacity and nutrients. Intense erosion also leads to soil structural decline and poor plant growth.

Soil erosion also has the potential for downstream impacts on creeks, rivers, reservoirs, lakes, and estuarine and marine environments. Waterborne erosion increases the supply of sediment to rivers. High concentrations of suspended sediments in rivers can:

reduce stream clarity;
inhibit respiration and feeding of stream biota;
diminish light needed for plant photosynthesis;
require treatment of water for human use;
smother the stream bed; and
increase land flooding.

Increased supply of sand and gravel from gully and riverbank erosion has led to deposition of sand and gravel beds (sand slugs). Sand slugs smother aquatic habitat. They can prevent fish passage, fill pools and other refugia and are unstable substrates for river bed life.

Soil erosion and sediment transport assessment

Soil erosion on agricultural land was assessed and placed in the context of river basin sediment budgets. Potential downstream impacts were identified and areas for continuing management attention are highlighted. These build upon major progress made through soil conservation activities to prevent erosion since the 1930s. The assessment includes improved prediction of sheetwash and rill erosion, and the first national assessments of gully erosion, streambank erosion, river sediment loads and deposition of sediment. Most significantly, the work is the first to explicitly relate patterns of sub-catchment erosion to downstream loads and export. The framework should prove valuable for future regional target setting and resource planning.

The assessment of water-borne erosion and river sediment transport covered:

sheetwash and rill erosion (also termed hillslope erosion potential);
gully erosion;
riverbank erosion; and
transport and delivery of eroded sediments through rivers and to estuaries.

Soil erosion in Tasmania

River basin name	Hillslope erosion as a % input to river	Gully erosion as a % input to river	Streambank erosion as a % input to river	Total sediment supply to rivers (t/yr)
Arthur River	4.19	2.29	93.52	38779
Coal River	34.08	43.14	22.78	32163
Derwent River	13.13	21.89	64.98	351478
East Coast	24.46	40.24	35.29	87713
Flinders - Cape Barren Islands	no data	no data	no data	no data
Forth River	12.18	38.41	49.4	16957
Gordon River	1.93	3.29	94.78	276625
Huon River	6.99	10.81	82.2	112375
King Island	no data	no data	no data	no data
King-Henty Rivers	24	.26	75.74	8337
Kingston Coast	31.92	32.04	36.04	1099
Mersey River	5.89	37.99	56.12	106546
Pieman River	12.04	1.75	86.21	37039
Piper-Ringarooma Rivers	4.56	46.68	48.75	85848
Rubicon River	2.62	60.75	36.62	15023
Sandy Cape Coast	.62	0	99.38	5570
Smithton-Burnie Coast	6.09	7.68	86.23	70736
South-West Coast	4.63	.04	95.33	193822
Tamar River	8.74	20.96	70.3	463606