RIVER MORPHOLOGY IN ENVIRONMENTAL FLOW ANALYSIS

An important consideration in environmental flow analysis is the need to determine the streamflows required to maintain the morphology of the river in a state consistent with the ecological needs of the river. The usual approach, if river morphology is considered at all, is based on the assumption that bankfull discharge is the ‘formative channel-modifying discharge’ that causes significant substrate movement. Another assumption it that the ‘effective’ discharge for moving substrate is generally less than or equal to bankfull discharge and that a conservative estimate of bankfull discharge is the annual maximum daily discharge with a return period of 1 in 2 years. These assumptions are too simple. In reality, all discharges are important and this concept is captured by a channel maintenance capacity index, CMCI. (The CMCI is the sum of daily discharges to a power when the daily discharge exceeds or is equal to a critical discharge based on the ability of the river to move the bed-material.) The CMCI is used to investigate alternative environmental flow requirements. A second element of analysis is to investigate the balance between a transport limited state and supply limited state in a river and how changes in water management may change the limiting state. The selection of an environmental flow requirement should consider the possibility water management may change the limiting state. Some rivers are transport limited for the larger sizes and supply limited for the smaller sizes. Quantile analysis is very useful in analysis in the process of investigating the limiting state. If the river is a gravel-bed river a third element is the needed to consider the possibility that that the size distribution of the bed surface material may change as a result of changes in the peak discharges in the river. The Cache la Poudre River is used as to demonstrate the analysis required to environmental flow requirements considering channel morphology. The limiting state (transport versus supply) as a function of the sediment size is investigated using data for the Green River in Utah. Data for Oak Creek in Oregon is used to investigate the possible change in bed-material size as peak discharges change.