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  • Determining the extent of braided riverbeds

    Posted on June 1, 2018 by in Canterbury Water Management Strategy, ECan, Events, Fresh water management, Land & Water Regional Plan, weeds

    In 2016, Philip Grove and Duncan Grey presented a report titled ‘Land use change on the margins of lowland Canterbury braided rivers, 1990-2012‘ to the Braided Rivers Workshop. The report, produced in 2015, pointed out that:

    “Across the region’s low plains, a total of 11,630ha of formerly undeveloped or forested river margin have been converted to intensive agricultural use between 1990 and 2012, an average of about 530ha per year.”

    It’s an eye-watering statistic. At the Braided Rivers seminar the following year, Philip Grove and Chris Keeling elaborated on the impacts of this ongoing conversion. It’s worth having a look at the PDF of their presentation (images below are from the report). There simply is no hiding the evidence. 

    However, as with most contentious issues, once you start digging into the details, you end up down a rabbit warren. While there are many opinions about what should and should not be allowed, before activities on a riverbed or river margins can be controlled or curtailed, the location of said river beds and margins first need to be defined, even in systems as dynamic and inherently unstable as braided rivers.

    Let’s start by taking a look at the RMA. Under its guidelines, a river bed is defined as:

    “...the space of land which the waters of the river cover at its fullest without overtopping its banks.”

    Okay… Does that mean the dry shingle of a braided river is not riverbed? So where are the banks? Either side of the active channels that happen to be flowing that day? Or the next day, when a fresh comes down and covers part or all of the shingle? Or three days later, when the channels have re-arranged themselves into entirely new places across newly created islands of now dry shingle? And what does ‘fullest’ mean? An annual fresh? A one in 5, 50, or 100 year flood? 

    As a geomorphologist, my default setting is to regard the entire braidplain as riverbed.  So the ‘banks’ must be the outer edges of the shingle and channels, right?

    Braidplain: from Grove and Keeling (ECan) ‘Addressing Land Use Changes in Braided Rivers’ Braided Rivers Seminar, 2017.

    But one look at the length of a braided river on Google Earth, and its clear that ‘edge’ is, pardon the pun, a fluid concept at best. The ‘edges’ of our braided rivers are everything from cliffs to wetlands, stop banks, levees, willows, countless weeds, forest, agricultural lands including centre pivot irrigation systems, and fences. When you factor in the bridges, dams, and abstraction of water for irrigation and power generation, the dynamics and flow regimes of most braided rivers have been irrevocably changed. So maybe we should put aside the past and consider where things stand at present. So where in this maze of landscape features and land uses are the ‘banks’? And as this is meant to help us plan for the future, we must also factor in the changing climate regime. NIWA are predicting more rain in the alps and less in the foothills, so foothill-fed braided rivers will likely be dryer for longer (sometimes months) while alpine-fed rivers are more likely to fill more often. And probably flood. Or not, depending on where you define the ‘riverbed’ and ‘banks’ in order to determine whether it’s ‘overtopping its banks’ or not.  Yep, the rabbit warren takes a U turn back to the initial question.

    Waiau River, 1997 from Grove and Keeling (ECan) ‘Addressing Land Use Changes in Braided Rivers’ Braided Rivers Seminar, 2017.

     

    Waiau River, 2015 from Grove and Keeling (ECan) ‘Addressing Land Use Changes in Braided Rivers’ Braided Rivers Seminar, 2017.

     

    The simple fact is, we’ve changed and still are changing these rivers at a rapid rate. So where to from here? ECan aims to navigate that rabbit warren by working with communities and stakeholders to develop an understanding of what values people place on our braided rivers, and just as importantly, why those values, be they environmental, cultural, social, and/or economic, are important to them. Understanding the ‘what’ and ‘why’ will help determine the extent of the ‘river bed’ in braided rivers as we collectively define it, and in turn, how to manage for those multiple values in braided river beds and their margins, in particular through RMA regulation.

    If you are invited to one of these meetings over the coming months, I urge you to attend. Working out the answers is not going to be easy, but it’s crucial if we’re going to have anything resembling the globally rare ecosystems we call ‘braided’ rivers in the coming decades.

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