The 40 years between AD 1875-1915 were the longest period during which the flow of the Truckee River was above the average.
(The 40 years between AD 1875-1915; were; the longest period during which the flow of the Truckee River was above the average) (the flow of the Truckee River; was; above the average; T:the longest period)
The 40 years between AD 1875-1915 the flow of the Truckee River
were was
the longest period during which the flow of the Truckee River was above the average
above the average T:the longest period