STREAM TERRACE PROFILES ARE STEEPER THAN THEIR FORMATIVE CHANNEL PROFILES: RESULTS FROM A HISTORIC SUITE OF TERRACES ALONG THE LOWER TRUCKEE RIVER, NV

The Truckee River in western Nevada has severely incised in response to a net 20 m of lake-level lowering of Pyramid Lake over the last 100 years, leaving a suite of cut terraces that extend about 15 km upstream from the lake where incision was arrested by bedrock in the channel. Channel planform changes and terrace development were mapped over a 70 year period using rectified aerial imagery and LiDAR data in ArcGIS. Discrete point elevations were extracted from the LiDAR data for channel remnants on each of the different-aged terraces and combined with channel distance measurements derived from the appropriate aerial imagery to derive true channel gradients for each of the photo years. These same point elevation measurements were also resolved to a common valley distance (CVD), as is often done when studying prehistoric terrace sequences.

Comparison of the true channel (TC) profiles to the CVD profiles demonstrates that the TC profiles are longer, less steep, and generally document greater incision per length of channel than do the CVD profiles. These differences exist because resolving terrace elevations to a CVD does not account for changing sinuosities and channel planforms through time. This is particularly evident when comparing the 1961 profile to the 1965 profile. During this brief four year period, Pyramid Lake dropped by about 4 m and two relatively large floods occurred. As a result, along its lowermost reaches the channel transformed from a single-stem meandering stream into a straighter braided stream that had clearly incised the 1961 floodplain, producing a cut terrace. The 1961 and 1965 CVD profiles, however, have the exact same gradient and there is no apparent incision, which is clearly not the case. Comparison of the 1961 and 1965 TC profiles documents that the 1965 TC profile is steeper and shorter and that stream incision reached a maximum of 12 m in that four year period.

The results of this study suggest that caution should be used when comparing the gradients of cut terraces formed in response to base level fall. Assuming that isolated terrace remnants can be correctly correlated, the gradients of the formative channels are less than the gradients of the terraces and the magnitude of incision separating different terraces should be considered a minimum when resolving terrace remnants to a common valley distance.