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Gilcrest/LaSalle Pilot Project Hydrogeologic Characterization Report valley is irregular and asymmetrical with approximately 380 feet of relief. This buried topography consists of a broad paleovalley incised by buried paleochannels, shown in Figure 7, and described below: Big Thompson Paleochannel — This paleochannel is northwest of the Study Area and aligns with the modern Big Thompson River valley, joining with the South Platte River paleochannel. South Platte Paleochannel — This paleochannel aligns with the modern South Platte River channel along the northwestern Study Area boundary. Gilcrest Paleochannel — This paleochannel extends from the southwestern Study Area corner, passes below Gilcrest and meanders under US Highway 85 until it joins the South Platte Paleochannel just northeast of LaSalle. Beebe Draw Paleochannel — This paleochannel aligns with the modern Beebe Draw valley joining the South Platte Paleochannel northeast of LaSalle. Tributary Paleochannels — these three paleochannels extend from dry washes descending from the highlands along the southeast flank of the South Platte alluvial valley about three miles southeast of US Highway 85, and continue below the alluvium to form a scalloped subsurface bedrock topography on the southeast side of the now -filled paleovalley. At the time of maximum incision, when the paleochannels were carved into the bedrock and prior to deposition of the alluvial valley fill, an ancestral Big Thompson River may have carved the northern Big Thompson paleochannel. At about the same time the ancestral South Platte River carved the Beebe Draw paleochannel (Smith 1964) and later the South Platte and Gilcrest paleochannels. Additionally, prior to the paleovalley filling with sediment, tributary paleochannels were incised by erosion into the bedrock channel walls, with landslides possibly causing the scalloped bedrock topography. Cross-section A -A' illustrates the deeper Gilcrest paleochannel which is offset from the modern river alignment (Figure 5). The deepest and most extensive paleochannel underlies the modern course of Beebe Draw. The presence of a deep bedrock paleochannel in Beebe Draw is consistent with interpretation by Smith (1964) of an ancestral South Platte River channel in Beebe Draw. Several tributary paleochannels enter the Gilcrest paleochannel from the south. Smaller tributaries along the southeastern portion of the main paleovalley create a scalloped edge to the buried topography similar to, and aligned with, the scalloped topography of the hills rising above the terraces southeast of the river valley. 10