Logjams needed for healthy streams


A lack of logjams in Australian streams and rivers created by deforestation could result in less habitat for aquatic animals and insects.

Research conducted in Rocky Mountain streams in the western United States that found streams full of dead trees were healthier than clean streams. The work, led by Dr Michael P. Venarsky from Griffith University’s Australian Rivers Institute, who is continuing his exploration of the findings that were originally published in Ecology of Freshwater Fish, has shown that dead trees create logjams within the streams, some up to 15m wide and 3m tall. Dr Venarsky, who conducted the research in his former role with Colorado State University, said these logjams drastically transform the shape of the streams from simple, single-channelled, fast-flowing streams, to streams with up to 19 separate channels containing both fast-flowing sections and pools up to 2m deep. “The large number of logjams in these streams produced more stream area, and thus more habitat for aquatic animals, with less water,” he said. “The stream with the most logjams had 19% more stream area than the stream with the fewest logjams, but this stream area was created with 65% less water.” The resulting increase in stream area caused a ~2.5 times increase in stream insect productivity and a ~6 times increase in the number of brook trout. Many streams throughout the western US have few logjams due to widespread deforestation – removing the source of dead trees – following European colonisation of the western US in the 19th century. “Mature forests are the critical supply of the large dead trees that create logjams, but forests in this region take greater than 200 years to reach maturity. Thus, natural recovery of logjams in these mountain streams is several generations away,” Dr Venarsky said. “Many Australian streams and rivers share a similar history to the Rocky Mountain streams that were the focus of these studies. “Widespread deforestation and vegetation clearing near streams has drastically reduced the amount of dead trees, and hence logjams, in many Australian rivers. Thus, the knowledge from these studies is likely relevant to many Australian streams and rivers.”