CTgeo
DURATION OF STREAM FLOW (modified by JGM from published texts) 


Streams are products of both the earth and the sky.   This short piece describes why the glacial features of Connecticut are so important to the flow of its streams and rivers.

Till is sediment that has generally been deposited beneath a glacier, in contrast to sediment deposited outside of or along the edge of the ice, called stratified drift.  Because it came directly out of the ice, till is a mixture of all grain sizes (poorly sorted) ranging from dust to sand to gravel to boulders, which allows the materials in till to pack together very tightly.  Consequently, water drains through it only slowly, and till cannot hold much water among the grains within it.  This is also called "hardpan" by non-scientists.  Stratified drift, on the other hand, is usually layered and graded by water outside of the ice, so its layers (strata) are well sorted into similar grain sizes.  Much of this is sand and gravel.  The good sorting allows more space between grains (porosity), and also those pores are better interconnected with spaces (permeable).  The high porosity and permeability of stratified drift allow water to flow through it, and also for much water to be stored in it (making a natural reservoir, or aquifer).  Till covers much of Connecticut's slopes and high areas, while stratified drift tends to be deposited in the valleys and lower, flatter areas.  Gardeners and farmers are often familiar with these materials!

As you should know, streams flow mainly from water that has collected from rain that has soaked into the uppermost sediment layers of the Earth (and also into cracks in the rocks beneath).  Streams may react quite differently to heavy rains, depending on whether the region around the stream -- its drainage basin --has mostly till or mostly stratified drift as sediment above the solid bedrock earth.  You might predict that rainfall runs rapidly off the surface of till rather than soaking in, much as it does off the pavement of developed urban areas, and so stream levels rise (and fall) rapidly in till regions.  Streams in stratified drift regions, on the other hand, are fed more gradually by water that is absorbed in great quantities by the strata, so the stream rises only slowly and flows more steadily, and for a longer period, as it is fed by the groundwater.  Which is the better situation?  Can we actually measure these differences?

Cumulative frequency curves, called flow duration curves, show the average percentage of time that specific daily flows are equaled or exceeded at sites where continuous records of daily flow are available.  A flow-duration curve for the Quinnipiac River at Wallingford (station no. 01196500) for the base period 1930-60 is one of the best known.  This station has the only long-term stream flow record in the report area.

Stream Fig1

A family of regional flow-duration curves developed by Thomas (1966), for ungaged sites, shows the effect of basin surficial geology on the shape of the curves.  Regional flow-duration curves based upon statewide data are shown in the figure.  In general, the curves show that streamflow from areas having a large proportion of stratified drift is more evenly distributed in time than streamflow from areas mantled largely by till.  This reflects the large infiltration and storage capacity of stratified drift and the resultant large proportion of ground-water runoff from these deposits.  In contrast, the uneven distribution of streamflow from till areas reflects the poor infiltration and low storage capacity of these deposits and the resultant large proportion of surface runoff.  The flow-duration curves shown in the figure above apply only to unregulated streams if their mean annual streamflow is 1.16 Mgd, or million gallons per day (=1.80 cfsm, cubic feet per second), the statewide average for the reference period 1930-60.  They may be used to estimate flow-duration.

Stream Fig2

This figure is a hydrograph, reflecting modification of stream response to precipitation following urbanization.  Peak discharge increases, lag time to peak decreases.  The original stream response was more similar to areas of natural stratified drift, while urbanization causes a response similar to areas of natural till cover.