Part of your understanding must come from watershed analysis, evaluating land use, sediment sources and sediment sinks (such as dams, low gradient sections).  Sediment movement can be episodic, but if high energy monsoons tend to erode more than aggrade, that suggests the supply of sediment becomes limited, and the channel materials with increased velocities are able to adjust and degrade to pass the higher flows.  If the braided system strongly stabilizes (anastomozed) with roots extending into channel, adjustments would be less than one would expect.  In an aggrading system, you might be able to carefully balance sand mining, so a more normal main channel system that will hold bankfull flow can be maintained.  The problem with braided systems may include riparian tree burial and mortality, able the unpredictable channel adjustments as the multiple channels are unable to sustain enough stream power to pass sediment or water efficiently.  But during monsoon, as mentioned, the remnant main channel system that has aggraded in past, may reestablish due to the increased flow velocity, rapidly eroding the channel (evulve something like a gully response), but during periods of lesser flows, the velocity is insufficient to pass sediment.  There may be measures to apply that might limit the degree of aggradation/degradation fluctuations if that is your intent, such as cross vanes or j hooks to reduce bank stress and provide grade and gradient control.  The ability to assess conditions and prescribe treatments is not easily accomplished through the stardard courses of my college days.  I still feel that the Rosgen training is best to help understanding, but may not be affordable or practical for you.  Rosgen did include a chapter in the USDA-NRCS Engineering Handbook, Part 650 on stream restoration that is on the internet.  Even with basic information, I would suggest you partner or interact with experienced hydrologist, soil scientist and possibly civil engineer that is used to going beyond the typical structural solutions.

Himolin,

I would say that any erosion is a threat, because once erosion starts it is difficult to control. However, river erosion is a complex issue that depends upon a lot of different variables, including soil/rock type, flow rate, particle transport size (clay, silt, sand, pebble, cobble), sediment load, vegetation, river maturity (grade), etc.

When can erosion be considered a threat? It strongly depends on what the adjacent land or the river itself is being used for. There is no single answer. River erosion can impact crops, buildings, roadways, etc. Too much erosion in a low hydrologic flow environment can cause siltation, thus preventing navigation, clogging water intakes for irrigation or drinking water, etc..

River dynamics are often not well understood, resulting in periods or episodes when the existing natural and man induced threats become a possibility or reality.  Rivers become more difficult to predict when they become unstable as either aggrading or degrading, especially so in substrates of fine materials or in areas with geologic instability or landforms.  Aggrading streams loose their channel capacity and result in increased flooding of adjacent areas, increased possibility of channel shifts or relocation, meandering or braiding.  Riparian vegetation in this circumstance may be buried in sediment causing some mortality and loss of support.  Degrading streams entrench into the landscape, loosing connectivity with floodplain, and stream power that was eased by flooding is focused into the processes of channel degradation and channel widening, often causing bank erosion or instability when the roots of riparian vegetation become undercut by bank scour.  Assessing this comes with training and experience, ability to both read indicators as well as collect and interpret data.  Dave Rosgen at www.wildlandhydrology.com has provided some pertinent references and training opportunities.  Coursework in hydrology, soils, geology and engineering helpful, but other functional expertise in reading the land and available remote sensing information can be helpful. 

There are ways to measure bank erosion through methods such as bank pins, channel and bank surveys, a series of LiDAR channel detail taken through time, or sometimes other remote sensing.  This subject is difficult to generalize to a specific circumstance, but with training and experience, understanding, many of the indicators will become easier to read and interpret by river professionals and those with a determined interest, who may come from a variety of backgrounds in some circumstances.  Evaluating changes relative to land use, riparian, stream and channel activities, climate change as well as natural dynamics of intensity is being recognized as difficult to predict with our limited data sets of typically decades, rather than centuries or longer.

But don't be discouraged, more than ever before are there increasing tools and information that should help those willing and interested to make a positive contribution and develop assessment skills that will be reliable and meaningful to those that seek to find this information.  But too often, these skills are not sought out, they may not be required, and damages that could have been avoided or prevented may infrequently, and unfortunately become the news of the day.  Perhaps greater education of these issues is a path forward. 

River erosion is a fact, a natural process that can be altered by man.  It may always be considered a threat to life, property as a force that is difficult to control.  But it can become more of a threat especially in urbanizing and developing areas if we do not consider and apply our full expertise and tools to our activities and management.  That may include greater avoidance or restrictions on developing in floodplains and in areas of soil and river bank instability, and understanding if the river is unstable, added adjustments many be needed. 

Williams' bank line undercutting also can have other effects. Trees that topple into the river can settle to the bottom or drift downstream to eventually lie on the bottom. These often form a sediment trap that can create sandbars.

Anyone interested in dynamic rivers should check out the geomorphology of the Red River in Oklahoma, Texas, Arkansas, and Louisiana. Historically, bank line slumps created huge rafting along the river. One raft was 6 miles long and extended 40 feet thick. These dams caused multiple overflow channels, with sediments building up behind the dams. There can be ca. 900 AD Indian village on top of a 15m bluff and .5k downstream a 1500 AD Indian village eroding out of the bottom of a 20m bluff. I had a project north of Shreveport in which the Red River moved laterally 600m in 1.5 years.

Thank you Sir James and William... We all know that erosion is a threat. But what I am dealing with here is a braided channel which either erodes or deposits sediments on the river bank periodically. In one monsoon it may cause erosion and the next it may deposit sediments (accretion), like William Sir said the river dyanmics is not well understood.. Erosion in one period may or may not be compensated by accretion in the next period.

And ecologically, in a landmass like forest stand on such river bank which is actually sustained by such activity of the river, erosion threat may depend on the carrying capacity of the ecosystem; meaning erosion may not be a threat as it is govern by two factors: accretion replenishment and carrying capacity of the landmass/ecosystem.

So in a study period, if area of erosion was higher than that of accretion of a landmass say forest, how would you determine if erosion is a threat and if so, by how much degree (keeping in mind the accretion process and ecological carrying capacity)? Is there any suitable methods for determination of such activity in the mentioned conditions?

For instance, if 5 % of the study area has been eroded during a study period of few decades, can it be considered really a threat? If not then how much percentage of the area should be eroded to consider it a real threat?

Part of your understanding must come from watershed analysis, evaluating land use, sediment sources and sediment sinks (such as dams, low gradient sections).  Sediment movement can be episodic, but if high energy monsoons tend to erode more than aggrade, that suggests the supply of sediment becomes limited, and the channel materials with increased velocities are able to adjust and degrade to pass the higher flows.  If the braided system strongly stabilizes (anastomozed) with roots extending into channel, adjustments would be less than one would expect.  In an aggrading system, you might be able to carefully balance sand mining, so a more normal main channel system that will hold bankfull flow can be maintained.  The problem with braided systems may include riparian tree burial and mortality, able the unpredictable channel adjustments as the multiple channels are unable to sustain enough stream power to pass sediment or water efficiently.  But during monsoon, as mentioned, the remnant main channel system that has aggraded in past, may reestablish due to the increased flow velocity, rapidly eroding the channel (evulve something like a gully response), but during periods of lesser flows, the velocity is insufficient to pass sediment.  There may be measures to apply that might limit the degree of aggradation/degradation fluctuations if that is your intent, such as cross vanes or j hooks to reduce bank stress and provide grade and gradient control.  The ability to assess conditions and prescribe treatments is not easily accomplished through the stardard courses of my college days.  I still feel that the Rosgen training is best to help understanding, but may not be affordable or practical for you.  Rosgen did include a chapter in the USDA-NRCS Engineering Handbook, Part 650 on stream restoration that is on the internet.  Even with basic information, I would suggest you partner or interact with experienced hydrologist, soil scientist and possibly civil engineer that is used to going beyond the typical structural solutions.

Yes, braided streams are complex beasts. By their very nature a single tree falling into a channel can shift that channel and others that interact with it.

I must say, again, Dr. Green adds a historical perspective of reading the land relative to archeology investigations that most river analysts and hydrologists would benefit from.  I am not sure I mentioned geologists and GIS expertise, but also needed.  As mentioned earlier, it may take a team approach to best address your specific issues, and understanding the past reference conditions and land use change effects brings about a better understanding. Luna Leopold was one of several authors that did so much to try to understand the complex history of some river systems by analyzing sediment deposits.  Often as researchers, we do our investigations independently, but there are definately benefits of collaboration with experienced individuals, and also when possible integrating in those in training or less experienced.

William,

Thank you. I think most projects/research benefit from a multi-discipline approach. However, not only have I studied geology and geomorphology, but I have worked closely with those disciplines, as well as hydrologists, sedimentologists, and riverine (underwater) archaeologists. It also helps to have a lifetime of wandering and wading creeks and rivers in different environments in order to perceive the causes and effects of channel migration, siltation, scouring, etc.

Thank you for your valuable suggestions. It indeed requires a multi-disciplinary approach. I shall try to consult experts to find solutions.