ClimateChange Assessment Guide.pdf - University of Waterloo

E-8Guide for Assessment of Hydrological Effects of Climate Change in Ontariocommunity. During periods of stress caused by low flow conditions, poor water quality or high instream temperaturesin the Credit River, some species of fish are known to take refuge in the Caledon tributaries. Island Lake supports avery active fishery favoured by local sports fishers. Nevertheless, habitat preservation is required to maintain thefishery in this state. Aquatic habitat is threatened by any form of reduction in summertime low streamflow conditions.Low streamflow can stress the local fishery for the following reasons:• Lower water levels result in a decrease in habitat;• Incomplete wastewater assimilation results in degraded water quality;• Lower water levels and reduced stream velocity aggravate warming/high water temperature conditions; and• Lower instream dissolved oxygen levels will result, caused by elevated biological uptake rates, lowersaturation concentrations with warmer temperatures and less reaeration due to lower stream velocity.Water quality is a concern in the reaches downstream of the Orangeville WPCP (Water Pollution Control Plant)discharge. Acceptable levels of wastewater discharge are directly related to receiving streamflow rate and indirectlyto water temperature. Climate change could result in lower summer streamflow and will result in warmer water ingeneral and thus, assimilative capacity may be significantly reduced. At present, Island Lake provides most or all ofthe water required for assimilation of Orangeville WPCP wastewater. Low streamflow can be evaluated using lowflow statistics such as the 7Q 20 or flow frequency probabilities such as the flow exceeded 98 or 99% of the time.Higher flow rates can lead to instream erosion and stream channel instability. The existing hydrologic characteristicsof the subwatershed are not problematic in this regard and therefore it is desirable to maintain this condition into thefuture. High flow rates can also cause flooding. At present there are seven identified zones with flood risk in thesubwatershed under a regulatory storm (Credit Valley Conservation, 2008). It is desirable to maintain the hydrologicand hydraulic systems so that peak flow rates are not increased in frequency or magnitude. Subwatershed 19 has been partitioned into 36 catchments as shown in Figure 1-4. The streamflow model setupadopts this discretization and utilizes a single reach within each catchment to represent surface drains and streamsand to receive groundwater and surface water discharge.Figure 1-5 shows the assessment area considered for groundwater modelling. The model domain was designed toencompass the entire upper subwatershed area of the Credit River, and to extend to the natural boundaries of thegroundwater flow system as interpreted from the shallow and deep groundwater level contour maps. The modeldomain boundary lies largely outside the subwatershed boundary to minimize potential boundary effects on modelpredictions within the study area.