PHYSICAL CHA.RACTERISTICS OF TRINITY RIVER 



Trinity River is approximately 159 miles long and drHins a semi- 

 wilderness area of approximately 2,900 square miles. It is divided 

 into five more or less distinct sections which have a direct bearing on 

 the utilisation of the stream by spawning and juvenile anadromous fishes. 

 The uppermost 18 miles of the river from its source to Ramshorn Creek 

 are precipitous (gradient 222 ft./mile), the channel is narrow, gravel 

 riffles are very limited, and the bottom is covered with large boulders. 

 The 12 miles of river between Ramshorn Creek and Trinity Center traverse 

 a broad valley into which many small tributary streams enter. The stream 

 has a gradient of 58 feet per mile and meanders through wooded and pasture 

 lands wherever gold dredges have left the original terrain. Its f^hannel 

 is broad and gravelly with large extensive riffles alternating with deep 

 pools. From Trinity Center to North Fork, the gradient is less s©- ere 

 (15 ft./mile), water volumes are greater, and very extensive riffles 

 characterise the channel. Most of the spawning grounds of salmon are 

 located in this 60-mile section of streeun. Between its North and South 

 Forks the river passes through a rocky canyon 40 miles in length. Water 

 flows are concentrated and made turbulent and exceedingly rapid by the 

 narrow confines which typify this canyon. Gradients are more severe 

 (23 ft./mile) and volumes of flow, in relation to channel capacity, are 

 relatively great. The Trinity between South Fork and its confluence with 

 Kalmath River meanders (gradient 12 ft./nile) the length of beautiful 

 Hoopa Valley (29 miles) and is characterized by broad gravel riffles 

 alternating with large, deep pools. 



Run-off and Flow 



The run-off ohai-acteri sties of Trinity River are cfuite similar 

 to those encountered in most California streams. The great bulk of the 

 annual run-off occurs in winter, while summer flows are quite low (Figure 

 2 Table 1). At Lewiston gaging station the extremes of flow between winter 

 and summer are represented by a high flow of 40,300 second-feet recorded 

 on February 28, 1940, and a low flow of 2 3 second-feet recorded on July 30, 

 1924 (n. S. Geological Survey data). The average annual run-off is 

 1,106,454 acre-feet as measured at Lewiston, and 3,811,520 acre-feet at 

 Hoopa, about 10 miles above the mouth. 



"Bie run-off pattern of Trinity River at Lewiston varies widely from 

 year to year, although the seasonal wet-dry cycles occur during corre- 

 sponding periods each year. The voliime of annual run-off from the 

 drainage above Lewiston had a range of approximately 250,000 to 2,500,000 

 acre-feet, with a mean of 1,106,454 acre-feet, during the years 1912 

 through 1945, 



