Sunday, January 29, 2012

The Finger Lakes of New Zealand: Geomorphology post by Prof. Noah Snyder

Dear Environmental Studies Program community,

As most of you know, I am on sabbatical this academic year, and living with my family in Christchurch on the South Island of New Zealand. From time to time I send postcards from my life in the Southern Hemisphere. I will be back at BC around the end of June. Until then, I hope you all have a great spring semester.

Noah Snyder
Environmental Studies Program Director

Over the Christmas holidays, we went on car-camping roadtrip around the southern South Island. On the trip, we spent three nights each on Lake Te Anau and Lake Wakitipu. These are two of the dozen or so long (10s of kilometers), thin (<10 km wide) lakes in the region. They fill the bottoms of deep U-shaped valleys carved by glaciers over the past two million years, and they are finger lakes just like the ones in upstate New York where I grew up.

Glaciers and rivers cut valleys with characteristic shapes. River water flows quickly (a meter per second or more) in narrow channels. In steeplands, as rivers erode the bottoms of valleys the adjacent hillsides get oversteepened and tend to landslide into the rivers, adding sediment that the river must carry away before it can cut a deeper valley. Through these coupled processes, rivers cut V-shaped valleys—narrow channels at the bottom with steep hillsides on either side.

Glacial ice moves slowly (10s-100s of meters per year) in deep, wide channels. It is a much more viscous fluid that water. As it flows, the weight of the ice quarries blocks of rocks from the bed and sidewalls of the valley, resulting in the characteristic “U-shaped” valley cross section—cliff walls and a flat bottom—when the ice retreats. The physics of the flow means that glaciers can erode “overdeepenings” or places where the rock surface beneath the ice gets scooped out. After the ice retreats, lakes get left behind in these depressions, which can be sized from small cirque tarns to finger lakes to Lake Superior. Lakes can also form behind dams created by moraines—ridges of sediment pushed out in front of the advancing ice. Ponds called kettles form in depressions created when chunks of ice get detached from the main glacier during retreat.

Lake-rich landscapes are therefore a good indicator of prior glaciation. When you look at a map dotted with many lakes of different sizes, you can be sure that glaciers have been there. Northern Minnesota. Maine. Landslide deposits in river canyons can create lakes, but these generally do not last a long time, because the dam erodes away, sometimes resulting in catastrophic flooding downstream. The devastating 2008 earthquake in Sichuan province, China resulted in the creation of many landslide dams. Of course, humans build dams and create reservoirs behind them. Almost all of the large lakes in the southern and western United States are the result of dams. Certain tectonic settings can result in the creation of lake basins. The Great Salt Lake is an example of this, as are Lake Baikal in Russia and the lakes of the East African Rift. Lakes can also form in karst limestone and some coastlines, but none of these settings have the large number of lakes that glaciers leave behind.

Lakes are temporary features geologically. Rivers carry water, sediment, organic matter and dissolved minerals downhill to the ocean. Lakes interrupt this process. Sediment entrained by rivers deposits in the still water of lakes, creating deltas. Through time this sediment fills the lake basin. When it gets shallow enough for plants to grow, biologic processes start to take over and the lake becomes a wetland. The advance and retreat of glaciers (twenty times in the past two million years) maintains the lake basins by scouring out the sediment deposited by rivers during interglacial times (like now).

This brings me back to the finger lakes of New Zealand. During the Last Glacial Maximum (LGM), twenty thousand years ago, sea level was 125 meters lower than it is now, because of all the water stored on continents as glacial ice. At this time, the valley glaciers of the NZ Southern Alps calved icebergs directly into the Tasman Sea (they come close now—we visited Fox and Franz Josef glaciers on the west coast). These vigorous, steep glaciers cut deep U-shaped valleys below modern sea level, which are now occupied with sea water (in fiords), lakes, or sediment in places where rivers have filled the depressions. The Finger Lakes of New York are the result of similar process. The key difference is that instead of ice sourced from snowfall high the mountains, the source was a continental ice dome. The elevation gradient or slope necessary to make the ice flow was simply the surface slope of this dome; it thinned in all directions away from what is now Hudson Bay in northern Canada. At the LGM, lobes of this ice sheet carved out deep valleys in central New York state and a subsequent readvance about 12-13 thousand years ago left moraines that plugged the southern parts of these valleys (driving south from Ithaca, my hometown, you cross this moraine on the way to Spencer), leaving behind closed basins to be occupied by the Finger Lakes. It is important to note that in both the New Zealand and New York cases, the valleys are the result of a series of glacial advances over the past two million years—you don’t get spectacular finger lakes from just one glaciation.

Today, rivers flow into these lakes and deposit deltas, which is the river’s attempt to build a ramp across the interruption created by the lake. Deltas have a characteristic shape with flat tops of gravel just above and below the water surface (called the topset), a steep underwater face of sand deposited at the angle of repose (the foreset), and thinner layers of mud carried out into the deeper lake basin in suspension (the bottomset). In both Lake Wakitipu and Lake Kaniere, another, smaller glacial-origin lake we visited farther north during our Christmastime tour, we swam on the flank of deltas. These are pleasant places because the delta shape means you have a gravel beach at the shore (good for skipping stones as any Ithaca kid can tell you) and a steep drop off in the lake. Most of the great parks of the Finger Lakes, such as Taughannock on Cayuga Lake, are situated on deltas. The best example we visited in NZ was a stream entering Lake Kaniere, which had a delta small enough to see the whole shoreline from the ground. I enjoyed wading out to the edge of the topset and kicking sand avalanches down the foreset slope. From there it was easy to imagine the small stream building a ramp across the lake with each successive flood. Of course that process happens rapidly in a steep landscape like NZ, where tectonics pushes the rocks up into mountain ranges and rivers, glaciers and landslides carry them back down at about the same rate. In the Finger Lakes, without active tectonics, the rate of infilling is much slower.

The southern lakes of New Zealand. From GoogleEarth.
The Finger Lakes of central New York state, at the same scale as the NZ map above. Cayuga and Seneca lakes are similar-sized to the finger lakes of NZ, and reach similar depths (the bottoms are modern below sea level). From GoogleEarth.
Swimming in Lake Wakitipu. The cloudiness of the water is the result of dust blowing off the Dart River delta.
Part of the delta of Dorothy Creek in Lake Kaniere, near Hokitika.

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