Effect of flow manipulation on the biota of a lowland river

"June 2001".

Project Number: Effect of flow manipulation on the biota of a lowland river - M/03/5082, SB/1/43; LWRRDC Project UM027.

MDFRC item.

For associated publications see page 14 in this report - Communications Achievements for the Campaspe flow manipulation project.

Intense regulation of the flows in Campaspe River, northern Victoria, have been implicated in the extremely degraded fish fauna. An environmental flow experiment was designed, which involved a ' before' period, to assess current status, and an 'after' period, which would test the response of components of the biota to the passage of 25% of flow ('translucent dam' approach) outside the irrigation season. Comparisons were made with the less regulated Broken River. Response variables included adult fish, fish larvae, macro invertebrates and shrimp. An extended drought prevented the regulating storage, Lake Eppalock, from reaching the trigger level for environmental flow releases until one month before the project officially ceased. However, our results clearly showed that summer irrigation flows have substantially altered the macroinvertebrate community in the most regulated sections of the Campaspe, from one characteristic of a lowland river to one which resembles that of an upland stream. We do not believe, however, that this has been the primary cause of the virtual elimination of native species of fish from this river. Nor is there any evidence that the lack of high winter and spring flows, caused by storage operation, has resulted in the prevention of spawning of fish. Rather, we conclude it likely that river regulation has created unfavourable conditions for recruitment through an interaction between hydrology and habitat and food availability for the young stages of fish. This, together with barriers to fish movement and the infrequent high winter/spring flows have probably eliminated most species and prevented recolonisation. We suggest that there is an urgent need for the operation of the Campaspe River (and similar rivers) as irrigation conduits be changed to better reflect the requirements of riverine biota.