Trial Monitoring Program for a River Restoration Project: Ovens De-willowing

"June 2005".

Project Number: Trial a monitoring program for river health - M/BUS/60.

MDFRC item.

Riparian zones represent the boundary between aquatic systems and the surrounding landscape and are often areas of high productivity and diversity. Riparian vegetation is also a major determinant of stream condition influencing water quality, habitat availability and productivity of the river (Schulze and Walker 1997, Pusey and Arthington 2003, Baxter et al., 2005). It is, therefore not surprising that changes in the amount or type of riparian vegetation may have a major impact on the river community including the invertebrate fauna with loss of riparian vegetation being associated with changes in community trophic structure (Thompson and Townsend, 2003, Danger and Robson, 2004). One of the major impacts of changing riparian vegetation can be to influence stream temperature. This can occur in a number of ways; Firstly, incoming shortwave radiation is absorbed by the canopy reducing daily maximum temperatures; Secondly, long wave radiation emitted by the canopy reaches the water and offsets the outgoing radiation emitted by the water; and thirdly, the canopy provides a buffer to air movement, reducing the effect of evaporation and conduction and therefore increasing the daily minimum temperature. (Rutherford et. al 1997). Rutherford et. al (2004) found maximum daily temperature changes of ± 4°C immediately downstream (600 to 960m) from 40 to 70% changes in riparian shading, in small slow flowing second order streams. Temperature changes of this magnitude are likely to be ecologically significant. Water temperature influences growth, reproduction and disease resistance in adult fish and hatching and development of fish larvae (Pusey and Arthington 2003), and also affects growth and development of aquatic insects (Butler 1984). The effect of canopy shading is maximised in smaller streams with greater surface area to volume ratios. Figure 1 shows the predictions from the STREAMLINE model (Rutherford et al 1997). With increasing stream size the rate of heating from incoming solar radiation decreases (the rate of change of water temperature is inversely proportional to mean stream depth). Similarly, the equilibrium water temperature varied inversely with stream order (Rutherford et. al 1997). This project undertook an examination of the short term effects of willow removal on both the stream temperature and the macroinvertebrate community in the Ovens River.