Ngā Pūrākau o Te Whenua – Kaitorete Spit

Satellite images, DEMs and hillshades are used to help tell the story of Kaitorete Spit and Te Waihora/Lake Ellesmere.  Thanks to Lloyd Carpenter for guidance on this post’s title, roughly translating to “A Landscape Story”.  Ngā mihi e hoa!

Oh great…  Yet another featureless, boring Canterbury vista…(nice day, though)

You could certainly be forgiven for thinking this!  We’re on the Little River Rail Trail looking roughly southeast.  Te Waihora and Kaitorete Spit are just over our right shoulder and parts of Banks Peninsula are visible to the left.  But I would argue that we’re right in the middle of a fascinating, geomorphic story here – it’s just that it’s not very apparent from where we stand.

Satellite imagery and aerial photographs give us unique perspectives on the world. They are full of things that may not look look like much on a human scale, but take on very different meaning when viewed from higher altitudes.  Increasingly, LiDAR is doing the same.  Here are a few examples to get us started.

This might just look like a wind blown high desert at ground level:

but when viewed from altitude (~500 m), well, the Nazca Lines are pretty amazing:

Switching continents, here’s a view from the ground somewhere in the Mauritania:

But just look at it from an altitude of around 100 km:

This is the so-called Eye of the Sahara, known as the Richat Structure to geology geeks.  Once thought of as an impact crater from an asteroid, it’s now believed to be the eroded remnants of a geological dome, sort of like a ginormous Moeraki Boulder being eroded away from above.

We are like the fleas on the elephant’s back, never quite getting the big picture from our squat perspective.  And we don’t have to go too far to see some similar things in our little part of Aotearoa.

Flying in and out of Christchurch, one can’t help but notice the patterns like these on the south side of the Waimakariri (at least I can’t), reminders of the meanderings of the river before the days of stopbanks:

(The whole of Canterbury Plains has basically been river bed at one point or another so one needn’t go far to see these patterns.)

Here’s another one.  Let’s start with the view from the ground:

Move along folks, nothing to see here.  We’re looking northwest just off highway 75 on the way to Little River (at the blue dot below).  Wairewa/Lake Forsyth is at our back and Te Waihora/Lake Ellesmere is not far away to the west.

Let’s jump 3,000 metres up:

What’s with those lines on the ground?  They weren’t particularly noticeable at ground level but are much more apparent from above.  Wet versus dry soil?  Tracks from the centre pivot?  Are they real?  By that, I mean are they more than just coloured areas on the ground from cropping perhaps?  Let’s use some data to check it out.

Here’s a hillshade layer derived from 1 m LiDAR data:

Going by the hillshade, these features are real enough that their slightly higher elevations are sufficient to create simulated shadows.  Querying the elevation data, these ridges certainly look real.  Using the Profile tool I can whip up an elevation profile chart:

(On this profile north is at the left, south to the right.)

And this isn’t the only place these occur.  Moving north and west, we see some interesting shelf-like arcs in Birdlings Valley (note that the curving state highway is clearly visible in the data to the left):

Looking east from the highway, here’s what that area looks:

And a profile:

Doesn’t look like much but that tree-covered ridge is close to five metres high!  And on the inside of Kaitorete Spit, some even crazier, arc-like patterns:

(The blue dot shows where the picture at the start of this post was taken.)

And real enough to be picked up by the hillshade:

But here’s what that area looks like at ground level from the rail trail (looking west):

Not much to see.  Taking a wider and higher angle we can start to see a bigger picture emerge with all sorts of lines and patterns:

These patterns aren’t new and they have been studied before.  A 1970 thesis by John Armon mapped these lines and ridges:

This figure was adapted from Armon, 1970 by Soons et al., 1997.  

These features are all quite real and tell a story, a geomorphic story, about the formation of Te Waihora and the Kaitorete Spit – the two go hand in hand, echoes of their shared history.  To get the full story we need to go back in time a bit, about 8,000 years.

Disclaimer: I’m not a geomorphologist (nor do I play one on television) and have relied heavily on some good chats with Peter Almond and papers from Jane Soons and Jamie Shulmeister for what follows.

Back in the day, what is now Te Waihora was an open bay, something like this (please excuse the modern features on the basemap):

Approximate coastline  early Holocene (adapted from Armon, 1970)

Eroded shingle from the Southern Alps was carried down to the sea bythe Rakaia and Rangitata Rivers, ending up in the ocean where it was exposed to the Southland Current, which runs southwest to northeast along the coast.  Most of this sediment started migrating north along the coast, transported along by longshore drift.  This started building a gravel spit moving gradually to the northeast:

Spits like this are long and thin with a rounded head shaped by wave action, (see, for example, the Brighton Spit, or the Boulder Bank in Nelson).  We see a similar shape at Southshore where the local current moves north to south:

Slowly, the spit grew closer to Banks Peninsula:

As it grew, it left behind the swirling hooked ridges we saw in the image above:

Once contact was made with Banks Peninsula, the longshore drift sediment continued to flow, gradually filling in the bays and building the beach ridges we saw above at Birdlings Valley and a few other locations, but also filling in the valleys and depositing sediment towards the south.

But that’s not the end of the story, no, because at a point in time around 700 to 1,000 years ago (Soons et al., 1997), the Waimakariri shifted (avulsed) to a new bed and flowed into what was the relatively newish Te Waihora lagoon, likely breaching the spit close to the current state highway and maintaining an opening to the sea.

As braided rivers are wont do, it later avulsed again, heading north to its current bed where the stopbanks now have it locked in.  This significantly reduced outflows through the spit, which was then able to reattach itself to the peninsula, permanently this time.  Longshore drift continued to relentlessly transport sediment which slowly built up and out, pushing the coastline further south:

And forming the Kaitorete Spit as we know it today:

And so, with time, we come to the current configuration of the Kaitorete Spit and Te Waihora as we know it today.  In the process, Te Roto o Wairewa/Lake Forsyth was also created as the shingle built up and cut off its connection to the sea.  But as recently as the 1830s, waka and whaleboats were able to enter Wairewa from the sea (see Soons, 1998 for more thorough detail on this), meaning that Wairewa as a lake is quite a young feature.  As sediment steams along the coast, the shoreline has gradually reoriented itself along an almost east-west line but currently doesn’t seem to changing much (an interesting question in itself!   Where’s all that shingle going now?).

As most of the coastal geomorphologists will have been muttering under their breath, Kaitorete shouldn’t really be called a spit once the Waimak shifted and the reconnection to the peninsula.  The proper term would be barrier beach but the name Spit has stuck, and I suspect most geomorphologists wince when they hear that.

It’s a fascinating geomorphic story, methinks, and hopefully you’ve seen how available spatial data help us understand how it unfolded.  The use of imagery and hillshades (plus some elevation profiles) have helped to tell this story, bolstered by field data and some hard mahi.  And I guess it highlights yet another great thing about living in Aotearoa – everywhere you look, there’s an interesting story the landscape has to tell us.

C

Aside: I wanted to get a sense of any meaning or story behind “Kaitorete” for this post…it proved to be quite a challenge.  With Lloyd’s help, the best we were able to do by way of translation was  “eat parakeet” and also ‘”distress cry of parakeet” by Taylor (1944) (similar in Fletcher, 1929) but also as “eat while fleeing” by White (1887).  The Te Waihora Co-governance site tells us that ” Its ancestral name, Kā Poupou a Te Rakihouia denotes the function of the spit – which is similar to that of an eel weir, guiding eels into the mouth of the hinaki (eel trap). In this case, the eels are guided to the Taumutu area where the kōumu (eel channels) are constructed to catch migrating eels.”  Place names are important but not always straightforward.

References

Armon, J. 1970.  Recent shorelines between Banks Peninsula and Coopers Lagoon.  Unpublished master’s thesis. University of Canterbury

Soons, J., J. Shulmeister and S. Holt.  1997.  The Holocene evolution of a well nourished gravelly barrier and lagoon complex, Kaitorete “Spit”, Canterbury, New Zealand.  Marine Geology 138:69-90.

Soons, J. 1998.  Recent coastal change in Canterbury – the case of Lake Forsyth/Wairewa.  New Zealand Geographer 54(1):7-14.