1970: A routine dry year the new dam absorbed

1970 is the mildest drought here, and its value is documentary: it is the reference ‘dry year’ in the basin’s founding water-quantity study, and it shows the new regulation system doing its job.

Local government & infrastructure

• Total basin water requirements to satisfy all uses on Okanagan River (including salmon minimum flows) were estimated at ~244,000 acre-feet/yr under 1970 development; this defines the drought-year supply threshold. This inflow is equalled or exceeded in about three years in four, so lower-inflow years such as 1970 draw on carry-over storage rather than triggering curtailment. [High] (source)
• The Study’s cost estimates for drought infrastructure impacts (e.g. ~$112,000 to modify Kelowna bridge and ~$100,000 to lower irrigation/domestic intakes) are tied to a hypothetical extreme drawdown to 1116 ft, NOT to conditions actually experienced in 1970; lake levels below 1119.0 ft had never occurred at the time of writing. [High] (source)

Environmental flow & fish

• Through 1970, minimum in-channel and fishery flows were ‘normally met’: since completion of the Okanagan Lake Dam in 1956, releases met consumptive use and in-channel minimums, and in most years additional discharges were released to provide minimum fishery flows for sockeye salmon migration through Osoyoos Lake and up to Vaseux Lake Dam. A single-year drought like 1970 did not compromise this. [High] (source)
• The Study documents that flows falling to zero downstream, with fishery requirements unmet, occurred during the severe 1929-31 drought - NOT in 1970. This is the distinguishing severe-case reference, and should not be attributed to 1970. [High] (source)
• Osoyoos/Okanagan sockeye escapement was highly variable through this era (pre-1999 estimates ranged from ~1,382 in 1961 to ~113,323 in 1967); a specific verified 1970 escapement figure was not retrieved and is not asserted here. Escapement records back to 1961 exist in DFO sources but the 1970 value was not confirmed. [Low] (source)

Sources: obwb.ca · pmc.ncbi.nlm.nih.gov · obwb.ca · wateroffice.ec.gc.ca · dfo-mpo.gc.ca