Origin and large-scale erosional features of glacial-lake spillways in the northern Great Plains

Deglaciation of the northern Great Plains produced two basic types of fluvial systems: (1) depositional regimes produced by the action of glacial melt water that formed outwash plains and (2) highly erosive systems produced by the sudden, rapid drainage of glacial lakes that formed spillways. Spillways are huge, deeply incised trenches that display all of the characteristics of fluvial channels. Glacial lakes drained in weeks or months, and discharges of ∼10⁵ m³/s were attained at bankfull stage. Seven spillway systems studied were formed as a consequence of the sudden, rapid draining of glacial lakes; many other large valleys in the mid-continent area and other glaciated regions were probably formed by the same process.

Spillways consist of deeply incised inner channels commonly flanked by broad, scoured outer zones. Initial stages of flow produced shallow, anastomosing channels; continued erosion produced longitudinal grooves, streamlined erosional residuals, and surficial boulder-lag deposits. Inner channels developed by the enlargement of one or more centrally located longitudinal grooves.

The development of erosional residuals was initiated by the incision of shallow anastomosing channels. The interchannel areas were progressively modified from irregular, quadrilateral, or elliptical shapes to an equilibrium, minimum-drag lemniscate shape as glacial-lake drainage proceeded. Correlations between length and width, length and area, width and area, and length/width ratio and k (a shape parameter of the lemniscate loop) are very high for all erosional residuals studied, although only ∼20% of the residuals display an equilibrium lemniscate shape. Stable relationships between length, width, and area of erosional residuals are therefore established very early and are maintained throughout the erosional modification to the lemniscate shape. The only shape factor tested which can be used to quantitatively differentiate lemniscate from nonlemniscate erosional residuals is the position of maximum width, as measured by the ratio of the length from the lee to the point of maximum width to the total length, Xm/L. The mean Xm/L ratio for erosional residuals in a spillway segment is an indication of the relative duration of flood flows that produced the spillway.