Knickpoint analysis of streams in North Dakota, South Dakota, and Nebraska

Analysis of stream profiles, seismic reflection studies, engineering studies, and historical earthquakes indicates uplift in south-central South Dakota and north-central Nebraska over the last 5-10 ma. The purpose of this study was to determine if this uplift could be detected by constructing topographic profiles along, and in close proximity to several streams in this region. The Cannonball River, ND, was used to compare the streams in South Dakota and Nebraska to a stream beyond the affected area. Data for the profiles were collected from 7 .5 minute topographic maps as well as GPS surveys. Abrupt changes in slope (irregularities) were determined along the profiles based on visual inspection and spreadsheet calculation. The use of spreadsheet calculation assured consistency in identification of irregularities among profiles. Profiles constructed along and in close proximity to streams in the study area contain numerous irregularities. In general, the profiles in close proximity to the streams (land surface profiles) show more irregularities than stream profiles. This is partly related to the method of construction of land surface profiles. Examination of the profiles has shown that some of the identified irregularities are the result of lithologic changes while others are the result of tributary streams. However, the reasons for many of the irregularities, which may include recent uplift, are not apparent and need to be field checked. Future work should also involve collection of more data through GPS surveying, which would increase the data base from which profiles can be constructed

Resolving a One-Year Ecesis Interval for Alaska Paper Birch: Dating a Rockfall Event, Wishbone Hill, Southcentral Alaska

Numerous large boulders at the base of Wishbone Hill, northeast of Anchorage, Alaska, suggest a historic rockfall event and potential for future surface instability, putting lives and property at risk. The source of the rockfall-boulders is an exposed syncline with a cliff face composed of conglomerate. The age of trees growing atop boulders provides a minimum exposure-age of those boulders and, thus, the rockfall event. To determine when the rockfall occurred, we dated trees growing atop the boulders using tree-ring samples collected from 30 Alaska paper birch trees. After mounting and polishing, each tree-ring sample was dot-counted, and tree-ring widths were measured using Measure J2X software to generate a master chronology (1938-2017). To estimate the youngest age for the rockfall event, we recorded pith-year for each sample. For samples lacking a pith (n=21), we used pith indicators to match existing rings to diagrams of corresponding ring widths, projecting approximate pith for each sample. All samples we corrected for sampling height (mean=0.8m) using a low estimate growth rate (0.6m/yr). The oldest birch tree sampled included pith and, with height correction, we estimate a germination year of 1936. When using first-year growth as an event’s temporal marker, accounting for the ecesis interval, the time between the availability of a new surface (i.e., boulders) and germination provides a more representative date of the event than using the pith/germination date alone. Considering birch ecesis and primary observations recorded in 1935, we propose that the rockfall event most likely occurred in 1934-1935. This finding suggests an ecesis interval as low as one year for Alaska paper birch in fresh rockfall areas. The risk of another destabilizing event may prompt those utilizing this area for recreational and residential purposes to reconsider future use