The Great Big Gravel Sponge

Operators Manual

What’s Groundwater?

• GW occupies the voids (pore spaces and fractures) in rocks and deposits below the water table.

• The water table generally parallels the surface contours, with hilly gradients.

• Usually the water table is deeper in uplands and shallow in low-lying areas (lakes, wetlands) – except in areas of high relief relative to the adjacent plains, like the Beaver Hills area

• GW moves very slowly through the surficial geological deposits (by percolation), directed by gravity downward or downhill (where a water table gradient exists)

• As a result, it can flow deeper underground, or laterally (e.g. toward deep riverbeds like the NSR)

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

Glacial Retreat• Retreat of the last glacier

created – literally – the groundwork for future landforms in the Edmonton region

• Glacial Lake Edmonton collected the meltwater left behind during the retreat of the main glacier –

• Lake was created by flooding of a pre-glacial river valley (Empress Formation)

• The stagnant ice east of the lake formed the Beaver Hills, depositing hummocky mounds of gravel and glacial till

Beaver Hills

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

Landforms Emerge• After the ice melted and waters

drained, surficial geological features emerged over pre-existing geological features (like the Empress Formation).

• Later, those soils and vegetation established over these features, but the underlying geology shapes the terrain and produces the physical landforms we see today

• It also influences how SW recharges GW aquifers through the sediment characteristics of surficial layers

Beaver Hills

Future valley

Gwynne channelSource: Godfrey, J.. 1993. Edmonton Beneath Our Feet.

Edmonton Geological Society

And Over Time…

• … our present landscape evolved.• With glacial till of varying composition (clay, sands,

gravels) and permeability over Cretaceous formations that now form our main GW aquifers

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

…And Left the Great Big Gravel Sponge

• The sandstone formations formed during the Cretaceous period are porous – and so can hold GW

• 2 key bedrock aquifers in the Edmonton area:– Belly River Formation– Lower Horseshoe

Canyon Formation

Beaver Hills

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

Great Big Gravel Sponge• The other main

aquifer in our area are the pre-glacial valleys of the Empress Formation – filled with sands and gravels – hold much GW

• Smaller tributary pre-glacial stream valleys feed into the Beverly Valley from the BH

Beaver Hills

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

Pre-glacialtributary

Great Big Gravel Sponge• The other main

aquifer in our area are the pre-glacial valleys of the Empress Formation – filled with sands and gravels – hold much GW

• Smaller tributary pre-glacial stream valleys feed into the Beverly Valley from the BH

Beaver Hills

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

Pre-glacialtributary

Lower Horseshoe Canyon Formation

• Deposited in a swampy delta sometimes flooded by sea– Contains numerous coal seams, which are fractured

in places – holds GW (particularly abundant under Cooking Lake Moraine)

– Upper part, of non-bentonitic sandstone also porous – holds GW (main supply for Calmar, Millet area)

• Water in these aquifers captured during Cretaceous period – Brackish – poor water quality– Recharged slowly by SW – through the overlying

sediments

Beaver Hills GW Flow

• Glacial till includes rock flour (now clay), plus pebbles, gravels, cobble – leaky sieve

• Higher permeability than Glacial Lake Edmonton – tight clay

• The hummocks of the BH formed by deposits of glacial till – relatively permeable in places

• Water table lies about 150 m above the NSR in the BH – close to ground surface (BH’s rise about 60 m above surrounding plains!)

• Combination of surficial sediments and water table position – great for recharge/discharge!

Great Big Gravel Sponge

• BH plays a key role in regional water cycle – from precipitation, to percolation into surficial sediments into underlying aquifers and preglacial valleys beyond its borders

Beaver Hills

Source: Godfrey, J.. 1993. Edmonton Beneath Our Feet. Edmonton Geological Society

Pre-glacialtributary