Gregory R. Brooks

The drainage of the Lake Ha!Ha! reservoir and downstream geomorphic impacts along Ha!Ha! River, Saguenay area, Quebec, Canada

Abstract On July 18–21, 1996, a severe rainstorm caused widespread flooding along the north shore of the St. Lawrence River, southern Quebec, Canada, particularly along rivers that drain the area just south of the Saguenay–Lake St. Jean region. At the Lake Ha!Ha! reservoir, inadequate available capacity to spill during the storm at the outlet dam resulted in the overtopping and erosion of a nearby earthfill saddle dyke. A new outlet formed at the site of the dyke and drained the reservoir over a period of many hours decreasing its area from 8.1 to 4.7 km2. Estimates of discharge range from 910 to 1380 m3 s−1 at the site of the eroded dyke to 1080 to 1260 m3 s−1 at a location 27 km downstream (about 8 km above the mouth of the river). The uncontrolled drainage of the Lake Ha!Ha! reservoir increased flooding along the lower 35 km of Ha!Ha! River where flooding was already in progress because of the rainstorm runoff. The flooding caused extensive geomorphic impacts along the river. Long sections of the river (totalling 25 km) experienced significant widening (locally up to 280 m) and channel incision (locally up to 20 m) while two reaches (6 and 4.5 km long) experienced up to several metres of aggradation. In general, the slope of the valley was the most important variable affecting whether or not the energy of the flow was above or below the erosive threshold of the valley bottom. Locally, a permanent channel diversion now exists where the drainage divide between the main river course and a small ravine was overtopped and extensively eroded. Communities, infrastructure, and industry located along the river were extensively damaged by the flood waters. The effects of flooding along Ha!Ha! River demonstrate that rivers on the Canadian Shield can undergo severe geomorphic changes caused by very high-magnitude flooding.

Dry Climate Disconnected the Laurentian Great Lakes

Recent studies have produced a new understanding of the hydrological history of North Americas Great Lakes, showing that water levels fell several meters below lake basin outlets during an early postglacial dry climate in the Holocene (younger than 10,000 radiocarbon years, or about 11,500 calibrated or calendar years before present (B.P.)). Water levels in the Huron basin, for example, fell more than 20 meters below the basin overflow outlet between about 7900 and 7500 radiocarbon (about 8770–8290 calibrated) years B.P. Outlet rivers, including the Niagara River, presently falling 99 meters from Lake Erie to Lake Ontario (and hence Niagara Falls), ran dry. This newly recognized phase of low lake levels in a dry climate provides a case study for evaluating the sensitivity of the Great Lakes to current and future climate change.

Holocene lateral channel migration and incision of the Red River, Manitoba, Canada

Abstract The Holocene evolution of the shallow alluvial valley occupied by the Red River was investigated at two successive river meanders near St. Jean Baptiste, Manitoba. A transect of five boreholes was sited across the flood plain at each meander to follow the path of lateral channel migration. From the cores, 24 wood and charcoal samples were AMS radiocarbon dated. The dates from the lower half of the alluvium in each core are interpreted to represent the age of the lateral accretion deposits within the flood plain at the borehole sites. The ages of these deposits increase progressively from ∼900 to 7900 and 1000 to 8100 cal years B.P. along each transect, respectively, from the proximal to distal portions of the flood plain. At the upstream meander, the average rate of channel migration was initially 0.35 m/year between ∼7900 and 7400 cal years B.P., then decreased to 0.18 m/year between ∼7400 and 6200 cal years B.P., and subsequently varied between 0.04 and 0.08 m/year. Net channel incision of the river since 8100 cal years B.P. is estimated to have ranged between 0.4 and 0.8 m/ky. The pre-6000-years-B.P. interval of greater channel migration is hypothesized to reflect a higher phase of sediment supply that was associated with the establishment of the river system on the former bed of glacial Lake Agassiz. Since 1000 years B.P., the outward migration of the meanders has caused a gradual enlarging of 0.7–2% in the cross-sectional area of the shallow valley at the two meanders. When considered proportionally over timescales of up to several centuries, the widening of the valley cross-section is very low to negligible and is deemed an insignificant factor affecting the modern flood hazard on the clay plain.

The 1996 Lake Ha! Ha! breakout flood, Quebec: Test data for geomorphic flood routing methods

This paper describes a set of field data suitable for the testing and comparative assessment of geomorphic flood routing methods. The data pertain to a particularly severe and unusually well–documented flood event: the Lake Ha! Ha! breakout flood of July 1996 in the Saguenay Region of Québec. In this event, heavy rains combined with the incision of a new lake outlet caused a major flood, which significantly reworked the downstream valley. Published and unpublished data from multiple sources are assembled and co–registered in a common frame of reference. These data include vertical and oblique air photos, hydrological records, surface geology information, and digital terrain models of the pre– and post–flood valley topography. The spatial coverage encompasses the drained lake as well as the full length of the downstream valley. To meet the respective needs of two– and one–dimensional approaches, the topography is sampled on a Cartesian mesh as well as interpolated along evenly–spaced cross–sections. The data set described in the paper is provided in digital form in the electronic supplement to this special issue.

Diatom and thecamoebian signatures of Red River (Manitoba and North Dakota) floods: Data collected from the 1997 and 1999 spring freshets

Overbank deposits that aggraded during the severe and moderate Red River, Manitoba and North Dakota floods of 1997 and 1999 were examined to characterize the typical diatom and thecamoebian flood assemblages. The deposits contain diatom assemblages dominated by Nitzschia spp., Stephanodiscus spp. and Navicula spp. as well as large quantities of Hantzschia amphioxys, Luticolaimbricata and L. mutica and lesser amounts of Aulacoseira italica, A. granulata, A. ambigua, Cyclotella meneghiniana, Amphora montana and Synedra ulna. Samples collected from the Red River in late summer are dominated by the plankton Cyclotella atomus, species of Stephanodiscus, Cyclostephanos sp. 1 (cf. delicatus), Cyclotella meneghiniana, Nitzschia reversa and Nitzschia acicularis. Late summer, floodplain lake plankton are dominated by Fragilaria capucina, Cocconeis placentula, Stephanodiscus parvus/minutulus and Cyclotella atomus. Red River thecamoebian flood assemblages are dominated by Cyclopyxis spp., with lesser amounts of Centropyxis aculeata and Arcella vulgaris. Late summer water samples from the Red River contain very few thecamoebian remains. The majority of the taxa present are Centropyxis aculeata, Arcella vulgaris and Cyclopyxis spp. Floodplain lakes are dominated by benthic thecamoebians, especially Difflugia oblonga and Cucurbitella tricuspis, a eutrophic indicator. The results of this baseline study suggest that diatom and thecamoebian assemblages can characterize flood deposits, within floodplain lake basins, during both very high and moderately high flood years, and that these assemblages are inherently different from those typically found in lakes along the Red River.

Vs30 and Fundamental Site Period Estimates in Soft Sediments of the Ottawa Valley from Near-Surface Geophysical Measurements

Seismic techniques have been used to supplement geological and geophysical borehole data for assessing earthquake hazard in the Ottawa Valley near Ottawa, Ontario, Canada. The methodology used to obtain critical parameters for site effect studies (depths of major contacts, shear-wave velocity-depth function) is presented in this paper. Bedrock depth values and basic stratigraphic information were obtained from water well descriptions and three geological units were defined: post-glacial marine, deltaic and fluvial sediments (mainly silt, clay and sand), glacial sediments (till, glaciofluvial sand and gravel), and Paleozoic (limestone, shale) and Precambrian bedrock. In order to augment existing knowledge of bedrock depths and overburden stratigraphy, 100 P-wave reflection soundings (test sites) were acquired. Good quality, high-frequency data have allowed identification of the reflections associated with the glacial-post-glacial boundary as well as the top of bedrock at each site. Subsequently, P-wave and...

Influence of loss of gradient from postglacial uplift on Red River flood hazard, Manitoba, Canada

The north-flowing, low-gradient section of the Red River in Manitoba has lost-60%/o of its valley gradient since 8 ka cal. BP. An existing hydraulic model of the modem Red River flood zone was used to examine the change in flood extent and depth of a discharge equivalent to the 1997 Red River flood (3970 m3/s) for scenarios of gradients at 8, 6, 4 and 2 ka cal. BP as well as 2 ka in the future. The modelling indicates a broad, shallow flood zone for all of the gradient scenarios, with extent and depth increasing over time. Between the 8 ka cal. BP and present-day scenarios, the flood zone increased from 1186 km2 to 1531 km2 (-29%/o) with depth increasing along four east-west cross-sections by 0.69 m (-61%), 0.91 m (-82%), 0.56 m (-64%) and 0.48 m (-86%). The flood extent and depths increased by a further 18 km2 (--5%) and 0.04-0.06 m (2-5%), respectively, by 2 ka in the future. Most of these changes to the flood zone occurred between 8 and 2 ka cal. BP, reflecting an exponential loss of gradient. A rise in flood depth equivalent to that which occurred between 8 ka cal. BP and the present-day, is assessed as increasing the long-term flood hazard; in contrast, the slight rise in depth between the present-day and 2 ka in the future does not.

Optical dating studies of mud-dominated alluvium and buried hearth-like features from Red River Valley, southern Manitoba, Canada

Optical dating studies of samples of late-Holocene mud-dominated alluvium, and heated alluvium in features interpreted to be hearths, collected from the banks of the Red River, southern Manitoba, Canada, were undertaken to assess the viability of these deposits as chronometers for floodplain evolution. Optical dating was done by utilizing the violet luminescence emitted during infrared excitation of the potassium feldspar in the fine-silt polymineral fraction. The alluvial samples, dated by radiocarbon to be about 1 ka old, yielded maximum limiting optical ages between about 1 and 5 ka, indicating that optical dating of such samples may provide useful chronological information in cases where material suitable for radiocarbon dating is lacking, or in cases where a radiocarbon chronology needs support. Furthermore, the surprisingly low residual optical ages from sediments of this type indicate that there is a high proability that sediments representing similar mud-dominated alluvial environments in the Pleistocene record will yield ages with adequate precision and accuracy. Bulk sediment extracted from the centre of a buried hearth yielded an optical age (1.4 ± 0.2 ka) that was broadly consistent with its associated calibrated radiocarbon age (0.23-1.05 ka), but good agreement (0.79 ± 0.09 ka) was achieved when small fragments of hard-baked mud (ñ0.5 g) extracted from within the feature were optically dated separately; hard-baked fragments from two other hearths yielded similar results. Optical dating of sediments from hearths formed in mud-dominated alluvial sediments thus represents a viable method for obtaining a useful chronology in circumstances where organic material suitable for radiocarbon dating is absent.

Flooding, structural flood control measures, and recent geomorphic research along the Red River, Manitoba, Canada

The Red River, Manitoba, Canada, is a low-gradient, meandering river that traverses the broad, flat Red River Valley on the north-eastern portion of the Great Plains of North America. The shallow stream-cut valley occupied by the river has insufficient capacity to contain large discharges, which allows higher magnitude flows to overtop the valley sides and spread up to 40 km across the adjacent clay plain. Major flooding impacts communities and rural areas, including the City of Winnipeg, and has caused significant flood disasters in the nineteenth and twentieth centuries. Since 1950, an array of structural flood control measures has been constructed (and some later upgraded) to mitigate flooding, including two diversion canals, a flood control dam, dyking (linear and ring dyking), and elevated earthen pads under structures. Multidisciplinary research initiated following the 1997 Red River flood provided a geomorphic context to the flood problem in support of decision making towards enhancing the flood-protection infrastructure. Based on flood signatures in the growth rings of bur oak trees (Quercus macrocarpa Michx.), the historic flood-of-record in 1826 is interpreted to be the largest Red River flood since at least 1648. An assessment of the decrease in river gradient arising from regional differential uplift revealed that the broad, shallow flood character is intrinsic to the landscape of the Red River Valley and that the contemporary rate of uplift is causing an insignificant change to the extent of flooding. An investigation of the evolution of the genetic floodplain indicates that fluvial geomorphic processes are not significantly enlarging or infilling the shallow stream-cut valley at rates relevant to altering the modern flood problem. Although flood management along the Red River is heavily dependent on structural measures, the design discharge of the integrated flood control works protecting Winnipeg has recently been enhanced to an up to 700-year return period, which reduces the flood hazard substantially.

Prehistoric Sensitive Clay Landslides and Paleoseismicity in the Ottawa Valley, Canada

The ages of 39 large retrogressive landslides and three areas of disturbed terrain in the Ottawa Valley were compiled using 122 published and unpublished radiocarbon ages. The chronological dataset includes 15 confined-valley landslides, 23 scarp-side landslides, and four special case features (one massive landslide, three disturbed terrain areas). The ages of the features range from ‘modern’ to 8,000 14C cal BP. Distinct clusters of 10 and 11 coincidentally-aged landslides at ~1,000 and ~5,150 14C cal BP are the groups of landslides previously interpreted to have been triggered by paleoearthquakes. Scarp-side landslides with scars between 0.1 and 10 km2 are the dominant failure morphology forming the two age clusters and constitute an important component of the interpreted paleoseismic evidence. Five of the confined-valley landslides are part of the ~1,000 14C cal BP cluster, but the other ten failures are of widely varying ages. One of the special case features, a massive landslide originating from a source area of ~20 km2, falls within the ~1,000 14C cal BP cluster. Radiocarbon ages representing the age of the Treadwell and Wendover disturbed terrains, suggest that these areas are contemporary with the Lefaivre disturbed area at ~7,900 14C cal BP, but this is not an unequivocal interpretation. Notwithstanding sampling bias within the dataset, it is inferred from the high proportion of dated landslides falling within the two interpreted paleoearthquake clusters that there is a ‘strong’ paleoseismic signature within the temporal pattern of landsliding within the Ottawa Valley. Based on the Ottawa Valley dataset, scarp-side landslides with preserved debris fields and scars greater than 0.1 km2, and landslides in general with scars greater than 1 km2, are more promising targets than confined-valley landslides for paleoseismic studies.