downstream flow stops, then moves upstream 

 and stops, and then moves downstream again. 



Data Analysis 



River discharge and height of tide are inde- 

 pendent variables in the analysis of flow rever- 

 sal in the lower Columbia River. The average 

 monthly flow at Prescott is characterized by 

 high bimodal peaks that occur annually in the 

 winter and spring. The higher high tides typ- 

 ically occur during the same period. 



River Flow 



The flow of the Columbia fluctuated greatly 

 during our study. It averaged 6,768 cubic 

 meters per second (m^sec~0 or 239,000 cubic 

 feet per second (cfs) and ranged from 2,209 to 

 14,442 m^sec-' (78,000 to 510,000 cfs). High 

 flow occurred during June 1968 and May 1969; 

 these high discharges were associated with water 

 from melting snow in areas east of the Cascade 

 Mountains. High river flow in January (Fig- 

 ure 1) was associated with precipitation in areas 

 west of the Cascades. Low flows occurred dur- 

 ing September in both years. Average monthly 



flows exceeded 8,495 m^sec"' (300,000 cfs) for 

 only 5 months of the 24-month period; this fact 

 is important in that it was only when the flow 

 was below this level that reversal became sig- 

 nificant at Prescott. 



Tidal Height 



High tides ranged from 1.5 to 3.1 m (4.9 to 

 10.1 ft). Tides of 2.0 to 2.6 m (6.6 to 8.5 ft) 

 occurred most frequently. Tidal influence pro- 

 duces differences in water elevations in the Co- 

 lumbia River that can be measured upstream 

 as far as Bonneville Dam (river km 225.3, river 

 mile 140). 



Characteristics of Flow Reversal 



During the 2 years of the study, 646 flow re- 

 versals of 60 min or longer (Table 1) were ob- 

 served and related to high tides. Reversals 

 occurred most frequently in August, September, 

 and October — normally the period of lowest dis- 

 charge — and least frequently in May and June — 

 the period of highest river discharge. Tidal 

 heights at Astoria which produced flow reversals 

 ranged from 1.5 to 3.0 m (4.9 to 9.9 ft) . 



_I25- 



Highest flow at which 

 reversal occurred 



Figure 1. — Average monthly Columbia River dis- 

 charge, at Prescott, Oreg., April 1968-March 1970. 

 Frequency of flow reversals lasting more than 

 60 min and highest discharge at which a reversal 

 occurred are also indicated. 



Table 1. — Frequency of flow reversals (^ 60 min) measured in the lower Columbia River at Prescott, Oreg., 



April 1968-March 1970. 



313 



