Quantitative characterization and elevation estimation of Lake Lahontan shoreline terraces from high‐resolution digital elevation models

Abstract

Abstract. The precision with which the elevation of a feature, such as a terrace, can be measured depends on the characterization of the noise contaminating the measurement. A method for identification and extraction of terrace feature elevations is presented and the topographic noise, due to erosion, as well as measurement error, is quantified. Highresolution digital elevation models (DEM) are acquired at six wave-cut, volcanic bedrock terrace sites from around the highstand of paleo-Lake Lahontan in the western Great Basin. Local DEMs, which are tied to regional geodetic control, were acquired using conventional total station, rapid postprocessed and real-time kinematic Global Positioning System methods. The topographic signal is processed with derivative filters for geomorphic feature recognition and averaging for noise reduction. Results indicate that noise levels for identifiable features such as riser crest, knickpoint, and slope inflection point are statistically equivalent and on the order of 0.5 rn standard deviation. Averaging within topographic bins spanning -50 rn along terrace strike yields feature elevation estimates with standard errors on the order of 0.12 m. The mean bench window elevation (between the riser crest and knickpoint) has the lowest standard error and is systematically related to water level. Propagation of surveying, geoid estimation, and terrace feature elevation estimation errors indicates that displacements on the order of 0.5 rn may be resolved using these methods. Elevation estimate interpretation involves terrace development, degradation, and neotectonics, but this new methodology has significant advantages in studies of neotectonic or geomorphic processes using local terrace elevation measurements. IntroductionRecently, much scientific research has focused on the interaction of tectonic and geomorphic processes, [e.g., Merritts and Ellis, 1994] and has resulted in a recognition of the need for quantitative assessment of the formational, degradational, and tectonic processes which interact to produce observed landforms through time. As pointed out by Burke and Dixon [1988] and Dietrich et al. [1993], quantitative models for any of these processes are most sound when based on high-resolution topographic data. It is now possible to acquire topographic data The primary goal of this study is to develop and test a high-resolution method of making local terrace elevation estimates based on the geomorphology of remnant shoreline terraces. To this end, we have sampled topography from around the highstand shorelines of Pleistocene Lake Lahontan in western Nevada (Figure 1). These data provide the means for isolating coherent terrace features, which are related to the water level. The method developed here will have application to neotectonic and lithospheric flexure (isostatic rebound) studies which are based on analysis of vertical terrace deflections, as well as other applications where terrace geomorphology must be quantized at some local scale in order to study regional trends.Th...

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“…Although the knickpoint serves as the best analog for water depth, other elements of the shoreline terrace serve as a good paleovertical datum. The analysis of Lahontan terraces by Hare et al (2001) using GPS and total station measurements indicates that noise levels for the riser crest, knickpoint, and riser infl ection are statistically indistinguishable and have a standard deviation of ~0.5 m. In our study using TLS and GPS, an increase of sampling density by 5 orders of magnitude reduced the noise levels a factor of 10 to ~5 cm (Singleton, 2004).…”

“…Hare et al (2001) used static and real time kinematic (RTK) GPS measurements, locally combined with total station surveying, to develop the protocols needed to adequately characterize ancient shorelines in Lake Lahontan of west-central Nevada. We expanded upon these techniques by using differential GPS positioning and TLS to provide the resolution needed to compare differences in the altitudes of correlated surfaces.…”

“…The degree to which shoreline features vary spatially is a source of debate (Singleton, 2004;Hare et al, 2001;Adams et al, 1999;Adams and Wesnousky, 1998;Atwood, 1994). Adams and Wesnousky (1998) and Adams et al (1999) proposed that for a given water level of Lake Lahontan, the height of constructional shoreline features varied by as much as 2.6 m over distances of 4-5 km.…”

“…Similarly, Atwood (1994) reported variability from 0.2 m to 1.9 m of shoreline highstands above the still-water level for the 1980 lake level of the Great Salt Lake of Utah. In contrast, our application of the quantitative techniques outlined by Hare et al (2001) coupled with the ability to sample geomorphic surfaces at a density of several thousand measurements per meter squared allowed us to document that specifi c geomorphic features on wave-cut terraces display characteristic altitude continuity with a standard deviation of 0.19 m and standard error of 0.04 m over distances of 2.4 km.…”

“…Where characteristic elevations were taken from crests of constructional barriers (Adams and Wesnousky, 1998) and water-level indicators such as debris lines, erosional steps, gravel ridges and beaches, and vegetation lines (Atwood, 1994), significant variability is reported. In contrast, where wave-cut terrace surfaces are measured, vertical differences of only tens of centimeters or centimeters are recognized over kilometer distances (Hare et al, 2001;Singleton, 2004). In addition to the noisy character of constructional features, some of the elevation variation may stem from low-resolution methods of measurement.…”