Question

Several times during the past 2.5 million years, huge ice sheets (continental- size glaciers) formed and spread across large parts of Northern Hemisphere landmasses and then gradually melted away. a. How do you think the formation of ice sheets affected sea level? b. How would rivers flowing into the ocean have been affected as the ice sheets expanded? c. What kind of adjustments would these rivers make as the glacial ice melted?

Short answer

a. Sea levels dropped during ice sheet formation. b. Rivers encountered lower sea levels and altered paths. c. Rivers adjusted with increased flow and new channels as ice melted.

Step-by-step solution

Understanding Ice Sheet Formation and Sea Levels

When ice sheets form, they are made up of water that has been accumulated as snow and compacted into ice on land. This process effectively removes water from the ocean, lowering sea levels globally. Therefore, when large ice sheets formed during the glacial periods, they caused a significant drop in sea levels.

Impact of Ice on Rivers Flowing Into the Ocean

As ice sheets expanded, they would alter the landscape significantly. Rivers flowing into the ocean would encounter less water volume to flow into, as sea levels lowered. Additionally, the cold climate would increase the viscosity of water, potentially slowing river flows. The increased ice also redirected flow paths of existing rivers.

River Adjustments During Glacial Melting

As the ice melted, the rivers would experience increased water flow due to the addition of meltwater. Sea levels would rise as the meltwater returned to the ocean, leading to rivers possibly flooding adjacent landscapes. Rivers would also need to adjust their channels, possibly creating braided streams or meandering channels due to the changing landscape and increase in sediment load from glacial deposits.

Key concepts

Ice Sheets

Ice sheets are massive layers of ice that cover large land areas. They are mostly found in polar regions and can span over millions of square kilometers. During the past 2.5 million years, the Earth has experienced several glacial periods, where ice sheets expanded significantly across the Northern Hemisphere.
These formations are crucial in regulating our planet’s climate and sea levels. As ice sheets grow, they effectively store large amounts of freshwater. This process involves accumulating snow, which compacts into ice over thousands of years. Because this water is stored on land, it leads to a decrease in the ocean's water content. This is why large ice sheets have historically caused a drop in global sea levels.
It's fascinating how the expansion and contraction of these ice masses play such an essential role in global hydrological cycles. Their presence affects everything from ocean currents to temperature regulation.

Sea Level Changes

Sea levels are directly impacted by the formation and melting of ice sheets. When ice sheets form, they lock up vast quantities of water that would otherwise be part of the ocean. This removal of water leads to a fall in global sea levels.
Conversely, when these ice sheets begin to melt, the locked water is released back into the ocean. This, in turn, leads to rising sea levels. Such changes have profound effects on coastal regions. Land that was once above sea level may become inundated, altering habitats for both humans and wildlife.
Aside from the direct effect of ice sheet expansion and melting, other factors that influence sea level include thermal expansion of water as temperatures rise, and changes in ocean currents. Understanding these complex systems is key to predicting future sea level changes and planning for their impacts.

River Dynamics

The dynamics of rivers are closely linked to the presence and absence of ice sheets. As ice sheets expanded during glacial periods, they significantly impacted rivers that flow into the ocean. With sea levels dropping, rivers faced changes in their gradient and sediment supply.
Expanding ice sheets often redirected river courses, changing the existing landscape. The colder climate during these times also increased water viscosity, which can slow down river flows.
When the glaciers receded, rivers experienced an increase in flow primarily due to the influx of glacial meltwater. This required rivers to adapt and often led to alterations in their paths. They might develop meandering channels or become braided due to the higher sediment load carried by the meltwater. These changes are a natural response to the fluctuating environmental conditions brought about by glacial cycles.

Glacial Meltwater Effects

The melting of ice sheets has a profound effect on the environment, particularly through meltwater. As glaciers and ice sheets begin to melt, they release large amounts of freshwater into rivers and eventually the ocean.
One immediate impact is the increase in river baseflow, which can lead to flooding in certain areas. This meltwater also carries significant quantities of sediment. This additional sediment from glacial deposits can lead to a variety of changes in river geomorphology, such as building up riverbanks or creating new channels.
Furthermore, as meltwater enters the oceans, it contributes to rising sea levels. This not only affects coastal regions but can also influence marine currents and ecosystems. The increased freshwater input may alter the salinity and temperature of the ocean waters, triggering changes in weather patterns and biodiversity.