Catchment-Scale Controls on River Geomorphology

28 Jan. 2021 | Logan River, UT

This project involves a variety of morphometrics analyses of the Logan River Watershed.

Longitudinal Profile

Google Earth makes it easy to see the longitudinal profile of the river, shown in Fig. 3. River Geomorphologists use elevation profiles to determine controls on the total catchment through elements such as concavity (total shape of the profile) and knickpoints (small abberations or changes in concavity along the profile).

Base Level Controls

The lowest point to which the Logan River cuts is 1344 m, where it coincides with Cutler Marsh, which feeds into the Benson Marina.

From the longitudinal profile, it appears that a previous base-level control existed at approximately 1500 m, where the ancient Lake Bonneville extended into what is now Logan Canyon. Lake Bonneville was present between 32 and 14 thousand years ago. This base-level control is an obvious knickpoint in the elevation profile of the Logan River, and is also the location of Third Dam.

Mainstem Length

From Franklin Basin, the Logan River extends 82.3 kilometers (51.1 mi) to its base control, Cutler Marsh, in Cache Valley.

Concavity

Concavity is defined by the difference in height between the total elevation profile and the midpoint of the mainstem length:

Concavity = 2A/H

where A is the height between the profile and a line between the two end points of the profile and H is the total height of the elevation profile.

The following calculations show concavity for the Logan River:

1. Define a line between the endpoints of the river at (82.2 km, 2465 m) and (0 km, 1344 m)

2. Interpolate at the mid-point of the distance, 41.1 m

2465 - 1344 2465 - y

_____________________ = _____________________

82.2 - 0 82.2 - 41.1

y = 1904.5

3. Determine A from the profile elevation at midpoint (41.1 km, 1559 m)

A = 1904.5 - 1559 m = 345.5 m

4. Determine the vertical distance between each end of the profile, H

H = 2465 m - 1344 m = 1121 m

5. Calculate Concavity, C = (2A)/H

Concavity = (2*345.5 m)/ 1121 m = 0.62

The concavity of the Logan River is 0.62, indicating a concave shape in the longitudinal profile.

Knickpoints

Knickpoints are points in the longitudinal profile where concavity changes or slope decreases significantly. In the Logan River, some of the most obvious knickpoints are labeled in Fig. 4. Many of these knickpoints are controlled by anthropogenic influences on the river and watershed. These include man-made dams and the highway running alongside the river through the canyon.

Catchment Metamorphics

Catchment Length:

Catchment area, A:

646,562 sq. m

646.56 sq. km

Catchment Perimeter Length, L:

172,420 m

172.4 km

Circularity Ratio

Rc = A/Ac

where A is the Area of the catchment and Ac is the area of a circle with the same circumference as the catchment.

A = 646.56 sq. km

circumference, C = perimeter = 172.4 km

Ac = C^2/ (4*pi) = 172.4 km^2 / (4*pi) = 2365.18

A/Ac = 646.56/2365.18 = 0.27

Elongation Ratio

Et = A^0.5/L

Et = 646.56^0.5/172.4 = 0.15

Form Factor

Rf = A/L^2

Where the length of the catchment along its axis is L = 65.6 km, shown in Fig. 5.

Rf = 646.56/(65.6^2) = 0.15

This indicates the Logan River Watershed is more elongated than circular, which is evident in the shape of the watershed (see Fig. 5).

Catchment Relief

Catchment Relief, H = Emax - Emin

Emax = 2976 m, Emin = 1344 m

H = 2976 - 1344 = 1632 m

Relief Ratio

Rh = H/L

Rh = 1632 m / 65.6 km = 24.9

Drainage Density

Dd = L/A

where L is the total length of stream channels in the catchment, L = 194.88 km

Dd = 194.88 km/646.56 sq. km = 0.301

Drainage Pattern

The drainage pattern of the Logan River Watershed appears to be Dendritic, which is the most common drainage pattern in catchments. The tributaries join a main stem (The Logan River) at acute angles as it proceeds through the canyon into the valley below.

Stream Order