John W. Johnston

Climate‐driven shifts in quantity and seasonality of river discharge over the past 1000 years from the hydrographic apex of North America

] Runoff generated from high elevations is the primarysource of freshwater for western North America, yet thiscriticalresourceismanagedonthebasisofshortinstrumentalrecords that capture an insufficient range of climaticconditions. Here we probe the effects of climate changeover the past 1000 years on river discharge in the upperMackenzie River system based on paleoenvironmentalinformation from the Peace-Athabasca Delta. The deltalandscape responds to hydroclimatic changes with markedvariability, while Lake Athabasca level appears to directlymonitor overall water availability. The latter fluctuatedsystematically over the past millennium, with the highestlevels occurring in concert with maximum glacier extentduring the Little Ice Age, and the lowest during the 11thcentury,priortomedievalglacierexpansion.Recentclimate-driven hydrological change appears to be on a trajectory toeven lower levels as high-elevation snow and glaciermeltwater contributions both continue to decline.

Late Holocene Lake-level Variation in Southeastern Lake Superior: Tahquamenon Bay, Michigan

Internal architecture and ages of 71 beach ridges in the Tahquamenon Bay embayment along the southeastern shore of Lake Superior on the Upper Peninsula of Michigan were studied to gen- erate a late Holocene relative lake-level curve. Establishing a long-term framework is important to exam- ine the context of historic events and help predict potential future changes critical for effective water resource management. Ridges in the embayment formed between about 4,200 and 2,100 calendar years before 1950 (cal. yrs. B.P.) and were created and preserved every 28 ± 4.8 years on average. Groups of three to six beach ridges coupled with inflections in the lake-level curve indicate a history of lake levels fluctuations and outlet changes. A rapid lake-level drop (approximately 4 m) from about 4,100 to 3,800 cal. yrs. B.P. was associated with a fall from the Nipissing II high-water-level phase. A change from a gradual fall to a slight rise was associated with an outlet change from Port Huron, Michigan/Sarnia, Ontario to Sault Ste. Marie, Michigan/Ontario. A complete outlet change occurred after the Algoma high-water-level phase (ca. 2,400 cal. yrs. B.P.). Preliminary rates of vertical ground movement calcu- lated from the strandplain are much greater than rates calculated from historical and geologic data. High rates of vertical ground movement could have caused tectonism in the Whitefish Bay area, modify- ing the strandplain during the past 2,400 years. A tectonic event at or near the Sault outlet also may have been a factor in the outlet change from Port Huron/Sarnia to Sault Ste. Marie.

Quantifying Lake Athabasca (Canada) water level during the 'Little Ice Age' highstand from palaeolimnological and geophysical analyses of a transgressive barrier-beach complex

We combine multiproxy palaeolimnological and geophysical analyses of a barrier-beach complex to estimate the water level of a sustained Lake Athabasca (Canada) highstand during the ‘Little Ice Age’ (LIA; 1600—1900 CE). Palaeolimnological analyses on sediment cores from the lagoon behind the barrier indicate high water levels during the LIA, controlled by subsurface hydrological connection with Lake Athabasca. Key features in the LIA stratigraphic interval are sand laminations deposited by overwash events and low C/N ratios reflecting deposition of predominantly aquatic organic matter. Ground penetrating radar profiles of the barrier reveal a depositional transgression sequence composed of waterlain landward-dipping foreset beds and horizontal topset beds, overlain by aeolian deposits. Stratigraphic relations suggest that the LIA washover deposits in the lagoon formed as the barrier was actively translating landward, and were generated by high-water events on Lake Athabasca that overtopped the barrier. This indicates Lake Athabasca rose to at least the elevation defined by the contact between the waterlain and aeolian sediments in the barrier, which is >4 m above the historical daily average from gauged records available since 1930 and likely represents storm events during the highstand. Assuming a similar relation between daily average and maximum lake level as in the historical gauge record, our findings suggest that Lake Athabasca was on average 2.3 m higher during the LIA than present day. Extrapolation of this high-water plane into the adjacent Peace-Athabasca Delta indicates that 70% of the modern landscape was frequently and perennially flooded until very recently, consistent with palaeolimnological evidence from several lakes in the delta.

Reconstructing paleo lake levels from relict shorelines along the Upper Great Lakes

Shorelines of the upper Great Lakes include many embayments that contain strandplains of beach ridges. These former shoreline positions of the lakes can be used to determine changes in the elevation of the lakes through time, and they also provide information on the warping of the ground surface that is occurring in the Great Lakes after the weight of glacial ice was removed. Relative lake-level hydrographs can be created by coring the beach ridges to determine the elevation of basal foreshore (swash zone) deposits in each ridge and by obtaining radiocarbon dates of basal wetland sediments between ridges to generate an age model for the ridges. Because the relative-level hydrographs are the combination of lake-level change and vertical ground movement (isostatic rebound), the rebound must be removed to produce a graph that shows only the physical limits and timing of past lake-level fluctuations referenced to a common outlet. More than 500 vibracores of beach-ridge sediments were collected at five sites along Lake Michigan and four sites along Lake Superior. The cores showed a sequence of dune deposits overlying foreshore deposits that, in turn, overlie upper shoreface deposits. The base of the foreshore deposits is coarser and more poorly sorted than an overlying and underlying sediment and represents the plunge-point sediments at the base of the swash zone. The plunge-point deposits are a close approximation of the elevation of the lake when the beach ridge formed. More than 150 radiocarbon ages of basal wetland sediments were collected to produce age models for the sites. Currently, age models exist for all Lake Michigan sites and one Lake Superior site. By combining the elevation data with the age models, six relative lake-level hydrographs were created for the upper Great Lakes. An iterative approach was used to remove rebound from the five Lake Michigan relative hydrographs and merge the graphs into a single hydrograph. The resultant hydrograph shows long-term patterns of lake-level change for lakes Michigan and Huron and is referenced to the Port Huron outlet. When the age models are completed for the Lake Superior sites, a hydrograph will be created for the entire lake.

Palaeohydrographic reconstructions from strandplains of beach ridges in the Laurentian Great Lakes

Abstract The current temporal and spatial context of water-level change, drivers of change, and possible future scenarios of the Laurentian Great Lakes is controversial. Palaeohydrographs are being constructed from measured subsurface elevations of palaeo-swash zones and modelled ages in strandplains of beach ridges that are preserved in embayments along the lakes’ edge. More than 800 elevations and 200 ages have been collected from 15 strandplains to construct site strandplain palaeohydrographs. Palaeo-beach elevations from whole strandplains or sets of correlative palaeo-beaches within strandplains are then used to establish an outlet palaeohydrograph for each lake. Adjusting strandplain palaeohydrograph elevations to account for glacial isostatic adjustment and refining age models help define the outlet palaeohydrograph. Common basin-wide water-level patterns and changes in outlet location or conveyance can then be interpreted. Systematic patterns of elevation and geomorphic/sedimentologic properties in individual, groups and sets of beach ridges in strandplains suggest that long-term patterns of water-level change and sediment supply occurred on decadal, centennial and millennial scales. Outlet palaeohydrograph construction for Lake Superior revealed discrepancies between geological and historical rates of glacial isostatic adjustment. These differences are currently being investigated using new data from Lake Huron.