Fluvial sediments have been deposited by rivers or streams. Sediments deposited in a river or stream channel itself range from muds to silts to sands to gravel to large boulders. With increasing transport distance, the sediments in fluvial systems generally become smaller in size, more rounded in shape, and more mineralogically mature.
The first sample shown below is fluvial sand from the Yongchi River (Yongji River) near Dongjeom in eastern South Korea. Much of the Korean Peninsula is quite mountainous (it's a tectonic collision zone), with many sediment-laden, decent-sized rivers. The sand has not been transported far from its source rocks (folded & faulted Lower Paleozoic and Upper Paleozoic sedimentary rocks in the environs of Taebaek City & Taebaeksan Provincial Park). So, not surprisingly, the grains are angular to at best subangular in shape. The sand is dominated by lithic grains plus quartz.
Modern fluvial lithic-quartz sand from the western edge of the Yongchi River (Yongji River) at railroad bridge, <1 km north of Dongjeom railway station, ~15 km SSE of Taebaek City, far-southeastern Gangwon South Province, eastern South Korea. Microphotograph by Sara Beth Kopczynski.
Yongchi River (Yongji River) & mountainous countryside, eastern South Korea.
This next sand is from Washington State's Green River. It consists of angular to subrounded rock fragments and quartz grains. The river at this locality is in the Puget Lowlands, and drains mountainous countryside of the Cascade Range. The outcrops in the Green River drainage basin are principally early Cenozoic volcanic rocks & sedimentary rocks.
Modern lithic-quartzose fluvial sand from the Green River in Flaming Geyser State Park, southern King County, western Washington State, USA. Collected & donated by Megan Murray. Microphotograph by Sara Beth Kopczynski.
Here's some orangish-brown, coarse-grained, fluvial lithic sand from a dry creek bed at the head of Cartwright's Creek in the Broken Hill Block of New South Wales, Australia. The bedrock geology of the Broken Hill Block is dominated by Precambrian high-grade metamorphic rocks. The source rocks for Cartwright's Creek sand are in the Willyama Supergroup (upper Paleoproterozoic). The angular-subangular sand grains are composed principally of quartz, feldspar, mica, and metamorphic rock fragments (typically mica schist).
Modern fluvial sand from Cartwright's Creek, ~15-16.5 km NNW of Purnamoota (Leadville), NNW of city of Broken Hill, Broken Hill Block, far-western New South Wales, southeastern Australia. Collected & donated by Molly Tannian. Microphotograph by Sara Beth Kopczynski.
Here's another fluvial sand from Australia - this time from a dry creek bed in the South Australian Outback. Unlike the above three fluvial sands, this sediment sample has a fair amount of subrounded quartz grains. There's much subangular quartz as well, plus lithic sand.
Fluvial quartz-rich sand from a dry creek bed a little north of the old Mernmerna railway station, eastern side of the Three Sisters Range, South Flinders Ranges, southern Australia. Microphotograph by Sara Beth Kopczynski.
Mernmerna, in the South Australian Outback, showing dry creek bed (middle left to lower right). Behind the dry creek, the straight line (upper left to middle right) is a long-abandoned railroad.