# Do Delta lands Naturally Subside? **URL:** \ **Category:** Factual Questions\ **Created:** [May 18, 2015, 3:12pm UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305 "2015-05-18T15:12:16Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [May 18, 2015, 3:12pm UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305/1 "2015-05-18T15:12:16Z")
Along with Global warming comes the threat of rising sea levels. Years ago, the island of Trinidad government was concerned, because their coastal beaches appeared to be washing away. However, a geological investigation revealed that the land was actually sinking-this was ascribed to the fact that Trinidad sits on the delta of the Orinoco river, and the weight of the delta sediments was causing subsistence of the island. So, are large river deltas always sinking? The Mississippi Delta is enormous, and the river annually deposits millions of tons of sand and mud into the delta-is this sinking a big part of the subsistence, or is the erosion from the ocean currents in the Gulf of Mexico the biggets effect? New Orleans is now sinking at the rate of about 1 foot/century-is ths mainly because of the withdrawal of water beneath the city, or the susnsistence of the delta in general? How do you differentiate the effect of rising oean levels vs. sinking of the land? ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [May 18, 2015, 3:35pm UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305/2 "2015-05-18T15:35:22Z")
> [@ralph124c](#): > > Along with Global warming comes the threat of rising sea levels. Years ago, the island of Trinidad government was concerned, because their coastal beaches appeared to be washing away. However, a geological investigation revealed that the land was actually sinking-this was ascribed to the fact that Trinidad sits on the delta of the Orinoco river, and the weight of the delta sediments was causing subsistence of the island. So, are large river deltas always sinking? The Mississippi Delta is enormous, and the river annually deposits millions of tons of sand and mud into the delta-is this sinking a big part of the subsistence, or is the erosion from the ocean currents in the Gulf of Mexico the biggets effect? New Orleans is now sinking at the rate of about 1 foot/century-is ths mainly because of the withdrawal of water beneath the city, or the susnsistence of the delta in general? How do you differentiate the effect of rising oean levels vs. sinking of the land? The Mississippi River Delta is also suffering because sediment-control projects (meant to reduce the need to dredge the shipping channel) have severely reduced the amount of sedment that is deposited. The result is that the delta is eroding away faster than the land is builting up. Also, I don’t know if it’s on delta land, but Venice has also been discovered to be sinking. Much (all?) of the “Aqua Alta” problem is not due to rising sea levels, but falling land. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [May 19, 2015, 7:19am UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305/3 "2015-05-19T07:19:37Z")
No, its too much to say that the Delta must sink. What they mean is, that because it is a river delta, none of it is very much above river level. Therefore a small amount of sinking of land, or rising the water level, causes flooding and erosion. Land can equally rise faster than global warming sea level rises. This could cause a river delta problem in that the river changes course, there being nothing solid to stop it… .Its easy to rectify with a bit of effort, but the land users may have other ideas. (eg if they use the river to irrigate, they may destroy any banks built to keep the river on original path.) Why would the land sink ? 1. Just natural… continents move, volcanoes push up and stop doing so… ice weights down one end of the continent, and stops doing so … 2.activities/construction/event that affect the water in the aquifer (eg pumping up the water from wells… or the weight of buildings squashing down !) can then cause the sediment at the aquifer layer to compact… the land sinks. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [May 19, 2015, 8:41am UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305/4 "2015-05-19T08:41:02Z")
> [@Isilder](#): > > No, its too much to say that the Delta must sink. No, it isn’t. The nature of delta sediments with their high organic load is [such](http://dspace.library.uu.nl/handle/1874/44586) that compaction is _always_ going to be an [important](http://www.earthmagazine.org/article/sinking-sediment-deltas-important-swelling-seas) [process](http://www.nature.com/ngeo/journal/v1/n3/full/ngeo129.html). So basically, it is practically inevitable with river deltas that they will subside. Especially given that modern activities again, almost inevitably, [act to reduce the historic sediment loads](https://books.google.co.za/books?id=9aE0qOyVNC4C&pg=PA73&lpg=PA73&dq=delta+compaction+rates&source=bl&ots=GPGsVpCQoM&sig=W75twIl_MQRnJ2B6stNPgdiPzgQ&hl=en&sa=X&ei=TfVaVZ7nCfTX7AaymYK4Bg&redir_esc=y#v=onepage&q=delta%20compaction%20rates&f=false), exaggerating the effect. Only a delta with normal or higher than historic levels of sediment deposition (and I’m talking the whole Holocene here) are _not_ going to subside. Can you think of _any_ such deltas? ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [May 19, 2015, 10:19am UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305/5 "2015-05-19T10:19:14Z")
> [@MrDibble](#): > > Only a delta with normal or higher than historic levels of sediment deposition (and I’m talking the whole Holocene here) are _not_ going to subside. Can you think of _any_ such deltas? On the scale of decades, watersheds that undergo major deforestation (usually anthropogenic, but I’m sure there are documented natural examples as well) undergo increased erosion, and therefore higher sedimentation rates and delta production. But on the scale of centuries or greater, your point is well taken; and even some of the deforestation-increased sediment examples have been partly or entirely offset by the engineered flow reductions you mentioned. ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [May 19, 2015, 12:55pm UTC](https://boards.straightdope.com/t/do-delta-lands-naturally-subside/720305/6 "2015-05-19T12:55:13Z")
> [@Diceman](#): > > Also, I don’t know if it’s on delta land, but Venice has also been discovered to be sinking. Much (all?) of the “Aqua Alta” problem is not due to rising sea levels, but falling land. Technically, Venice isn’t even “on land,” certainly not a delta. Its situation is unique.