Developing River Styles

To start off my analysis for this reach of the Diamond Fork River, I first got an overview of the riverscape I am working with by using Google Earth Imagery. I thought about questions like: What is the valley setting? What are the sediment inputs that I am seeing? Where is the active and inactive floodplain?

Reach C: Gravel-Bed, Meandering Reach

Site Length = about 1175 meters

Riverscape Width: downstream end = about 53 meters, upstream end = about 124.5 meters

To calculate slope, I found the elevation at the top of the reach (1553 meters) and the bottom of the reach (1545.5 meters) using Google Earth. The reach is 1175 meters in length along the bankfull channel centerline.

(1553 m - 1545.5 m)/ 1175 m = 0.0064 x 100 = 0.64 % slope

To calculate sinuosity, I measured the length of channel centerline (1176.6 m) and the length of the valley bottom centerline (1175.1 m).

1176.6 m/ 1175.1 m = 1.00 sinuosity which means that this reach is straight to very low sinuosity.

To calculated confinement, I measured the length of the channel margin (132m) and the length of the channel (1175 m).

The length of the confining margin = 59 m + 70 m + 43 m + 58 m = 230 m.

230 m/ 1175 m m = 0.196 x 100 = 19.6% confined so this reach is partly-confined planform-controlled. I originally thought this reach would be considered unconfined, however the math tells us otherwise...

Area of Valley Bottom Margin: 119,906.0 meters squared

Area of Active Channel: 22,126.4 meters squared

Area of Inactive Floodplain: 6,965.0 meters squared

Area of Active Floodplain: 90,814.6 meters squared

(119,906.0 meters squared - 22,126.4 meters squared - 6,965.0 meters squared = 90,814.6 meters squared)

Instream Geomorphic Units

Geomorphic units are the building blocks of river systems. They are defined by their morphology (shape and geometry), sedimentary composition, bounding surfaces, and position on the valley floor. Erosional or depositional processes, or a range of these, produce these features. Instream geomorphic units are found along a slope-induced energy and textural gradient. The pattern of these instream geomorphic units helps explain the geometry (shape and size) of the channel.

Based on the Google imagery, the fact that the slope is low, and that the channel bed is gravel throughout this reach, runs can be seen. This means that water and sediment is being conveyed smoothly and the slope of the channel is low to intermediate.

Because the channel bed is gravel, forced riffles will occur between bends in the channel. The flow here is characterized by high-energy turbulence over lobate accumulations of course bedload materials and wood.

There could also potentially be pools in this reach of the channel. Alluvial pools are alternating deep areas of channel along an undulating reach-scale longitudinal bed profile. Pools tend to be narrower than riffles and act as sediment storage zones.

There are a few lateral bars in this reach of the channel. Bar length and width are proportional to the flows in the channel. Bars form by lateral or oblique accretion processes, with some suspended-load materials atop, meaning they typically form in an upward fining depositional sequence.

There is also three point bars. Point bars result from lateral shift in channel position associated with deposition on the convex bank and erosion on the concave bank. The gravel bedload material is moved by traction towards the inner sides of the channel bends via helicoidal flow. The coarsest material is found to be deposited at the bar head.

All of these features are displayed in the images below:

Floodplain Geomorphic Units

Floodplains are areas of sediment accumulation made up of alluvial materials between the channel banks and the valley margin. Floodplains accumulate sediment when the sediment supply during overbank flow events exceeds the transport capacity of the flow and sediment is deposited. Floodplains are often poorly drained, acting as stilling basins in which fine-grained suspended-load sediments settle out from over-bank flows.

I concluded the presence of these floodplain geomorphic units based on the Google Imagery and the LiDAR DEM hillshade.

I thought these features were alluvial terraces, however after discussing this site in class, alluvial terraces are not found in the valley bottom margin. Alluvial terraces are initially formed by lateral and vertical accretion processes under prior flow conditions. A change in base level or shifts in sediment-load and discharge regime (linked to climate) prompt downcutting into valley floor deposits, abandoning the former floodplain. Below is what I thought was an alluvial terraces, but instead is probably just an elevated surface. This surface could have been part of an historical active channel.

Could potentially be floodplain sand sheets present. Floodplain sand sheets are associated with rapid sediment-charged bedload deposition on the floodplain during extreme floods events. Material is typically deposited on planar, homogeneous sequences. Sand sheets build the floodplain vertically.

There appears to be some impact from humans on the floodplain as there is a small road through the upstream end of the floodplain.

All of these features are displayed in the images below: