The Sacramento River flows through the wide, seemingly flat Sacramento Valley, but the land rises, imperceptibly, as you head north towards Redding. Up north, as the river leaves the mountains, the course begins to meander.
A meander is created by both erosion and deposition. The river flows fastest on the outer bend. Lateral erosion cuts into the side, creating a steep bank known as a river cliff. On the inner bend the current is slower, so sediment is deposited, building a slipoff slope or river beach.
As erosion cuts into the river cliff and deposition builds the slip off slope, the bend becomes more exaggerated. Over time, meanders grow in size and move across the floodplain. As the meander grows more curved, floods can cause the river to cut through the neck of the bend. The river channel then straightens, leaving the loop of the old meander cut off from the river.
This horseshoe shaped feature is called an oxbow lake. At first the oxbow lake contains water, but over time it may dry up to leave a meander scar.
Meanders and oxbow lakes are features that show how erosion and deposition work together to shape dynamic landscapes.
Today the river doesn’t move around as freely because people built levees, hardened the banks, and mined gravel starting in the 1800s. The oxbow lakes you see now formed before those changes, when the river could wander naturally.
The oxbow here, now called Old River, was cut off from the current channel sometime between 1947 and 1952. A USGS map from 1910 shows the river flowing through a huge u-shaped bend here. A 1947 map shows the river straightening out, and an island here. By 1952, the lake is completely cutoff from the current channel. At some point I think the county lines had to be adjusted to accomodate the shift in the river, since it defines the lines.
From Ground Zero here you can only see a small portion of the whole curved oxbow lake, but look at it on a map and you see the extent of the huge curve that once flowed here. On a recent flight into Sacramento I was lucky enough to have a good look at it from above.
Logging Requirements
1. **From GZ** -- Which shoreline was the outer (erosional) bank and which the inner (depositional) bank? Describe the clues - slope steepness, bank height, vegetation, or sediment texture.
2. **WP2** -- Is this oxbow an open lake, shallow marsh, reed wetland, or something transitional? What evidence supports your estimate?
3. **WP2** -- Estimate how far the river has moved from this part of the oxbow. Is there evidence that much has been done here to keep the river at bay?
Learn more
These resources can help you explore river science and the Sacramento River:
• National Geographic - https://education.nationalgeographic.org/resource/oxbow-lake/
• USGS - https://eros.usgs.gov/earthshots/oxbow-lakes
• The Nature Conservancy — https://www.nature.org/content/dam/tnc/nature/en/documents/Oxbow-Restoration-Toolkit.pdf
• Google Earth - Zoom out to see meander scars, scroll bars, and oxbow lakes across the floodplain of the valley.