While reading about the geography in a certain region I came upon this sentence,
“But these rivers are too fast-flowing to carry silt or sludge to the sea.”
Upon reflection I realize that the Lower Nile and Mississippi and Amazon are slow moving rivers that carry a lot of silt in them. But it’s not clear to me why a fast moving river wouldn’t also move silt and sludge downriver.
Why is this?
Hydraulics… they’re complex. See 5, Suspended Load
Hydraulic Structure,Equipment and Water Data Acquisition Systems - Volume II - Google Books
Tweed said tae Till:
‘What gars ye rin sae still?’
Till said tae Tweed:
‘Though ye rin wi’ speed
An’ I rin slaw–
Yet where ye droon ae man
I droon twa.’
Kipling
More relevant is that terrain has a preferred hydrological slope; if the river is too steep, it will erode the land, slowly making the river less steep…and if it isn’t steep enough, it stacks up silt, slowly making it steeper.
(Also, vaguely sad for tourists, Yosemite National Park’s “Mirror Lake” is well on its way to becoming a meadow.)
Your quoted sentence doesn’t make sense without the context of what they were talking about. It might be that the author was talking about the small headwater streams of a larger watercourse and was saying that the majority of the fine suspended load came from higher order slower flowing reaches downstream and therefore the fine load originating from the fast moving headwaters isn’t significant… but that’s just a guess.
Of course fast flowing water can carry fine suspended sediments just as well or even better than slow-flowing water; there’s no physical reason it couldn’t. I think the point of what every you read was that if you measure the total fine suspended load of the Nile or other large slow moving stream as it enters the ocean the percentage of it that load coming directly from the headwaters would be tiny, or put another way the small fast moving head waters aren’t contributing or moving the tons of suspended load; that’s coming from the lower (slower) reaches.
Exactly… what I recall from geology class is that if you were looking at a sedimentary rock, you could tell what sort of water course it came from by the size of the particles comprising the rock.
Larger particles = faster flow, since faster flowing water can keep larger particles suspended. Stuff like siltstone, for example, was from lakes or really slow flowing rivers, as they were going slow enough for those super-fine particles to settle out.
The implication is that faster flowing rivers can easily carry the finer particles. However, like mmmiiikkkeee said, the very fast flowing parts of rivers are usually relatively short, and near the headwaters, so it’s likely that the total load of particles is lower, since the river hasn’t flowed very far yet. Similarly, nearer the delta, it’s flowed for a long distance, AND likely had multiple tributaries also carrying heavy loads of silt merged in over its length.
LIKELY ??? All rivers can get squashed into a narrow river culvert, with a hard rock base that keeps it from getting deep…
What I am saying, is that the old saying “a river that runs slow runs deep” is not correct. A river that is wide OR deep can run slow. If its neither, its gotta run faster.
The OP must be talking about deposition. If its a running river, it doesn’t deposit fine stuff.
silt and pebbles, or sand and pebbles. Perhaps a river bed. When its very still only eg silt is carried and so only eg silt deposits. When its running fast, only pebbles remain behind. (could also happen with sand and pebbles.)
dirty sand stone - silt and sand, such as ocean bed near the river entry.
clean sand stone - made from pure sand, its the sand that formed into a sand dune eg a beach
mixed silt,sand,pebbles. Alluvial plain, where the river spreads out and slows down, creating a plain of deposits.
pebbles … fast flowing river
sparse rocks dropped out of geological context - perhaps a glacier or flood carried it.