| Historic Riparian Habitat |
As groundwater resources have been depleted over the years, riparian vegetation within the valley has undoubtedly changed. Gallery cottonwoods (Populus fremontii) may historically have been found within the river valley23. An analysis of historic aerial photographs of the river valley through time provided data as to how the decline of the water table has affected the distribution of riparian vegetation within the valley.
Four aerial photograph mosaics, from 1938, 1978, 1989, and 2005 were chosen for the historic riparian vegetation analysis. These images were geo-referenced to the 2005 NAIP aerial photograph27. Each aerial image had a resolution of 1:40,000 and were obtained in color infrared when possible.
The majority of natural vegetation within the valley subsists on natural precipitation. However, according to published literature from 1951, approximately 2,100 acres along the Cuyama River historically contained vegetation that relied on groundwater37. This vegetation reportedly extended downstream to Bee Rock Canyon, where highway 166 crosses the Cuyama River. Specifically, 1,650 acres were identified as tulles, cattails, and grass; 150 acres were categorized as dense trees, grass, and brush; and the remaining 300 acres were identified as sparse grass, brush, and trees37. |
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Image: Historic location of water-loving vegetation along the Cuyama River |
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| Qualitative Analysis
Visual inspection of historical aerial photographs revealed a significant loss of riparian vegetation concurrent with the expansion of intensive agriculture. The 1938 photograph depicts a region along the Cuyama River channel that contained identifiable riparian vegetation. In the photograph, riparian vegetation appears as a dark, mottled texture surrounding the river channel that is distinct from the lighter and comparatively smoother grassy uplands. In the 1978 photograph, patches of dark vegetation remain, but are conspicuously absent in subsequent years. As the agriculture intensified and encroached upon the river channel, much of the visible, woody riparian vegetation historically present along the Cuyama River was lost. |
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These images are courtesy of the Map and Imagery Laboratory, Davidson Library, University of California, Santa Barbara and are for educational purposes only. The flight_frame_year are listed by each photo.
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1938
(c-5140_6_1938)
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1978
(usda-40-06079-bw_278-116_1978)
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1989
(napp_1892-61_1989)
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2002
(napp-3c_12451-101_2002)
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| Quantitative Analysis
To quantify changes to the riparian zone over time, eighteen transects perpendicular to the river were established at approximately 1 mile (1.6 km) intervals along the river in 1938, 1978, 1989 and 2005 aerial images. Analysis of the river spans 27.5 km, and was centered in the portion of the valley currently utilized for intensive agriculture. Channel and woody vegetation in the active riparian area were measured across each transect using the measure tool in ArcGIS 9.3. For this analysis, areas dominated by trees or shrubs were classified as woody vegetation. Bare or sparsely vegetated areas were classified as active channel. In areas where the channel was braided, segments in each class were summed. |
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Image: Placement of transects in 1938 aerial image for riparian analysis. The airphoto image is courtesy of the Map and Imagery Laboratory, Davidson Library, University of California, Santa Barbara and is for educational purposes only. |
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Transect |
Distance (km) |
Transect |
Distance (km) |
1 |
0 |
10 |
15.1 |
2 |
1.83 |
11 |
16.4 |
3 |
3.27 |
12 |
18.0 |
4 |
4.20 |
13 |
19.9 |
5 |
6.61 |
14 |
20.9 |
6 |
8.95 |
15 |
22.4 |
7 |
10.6 |
16 |
24.1 |
8 |
12.3 |
17 |
25.7 |
9 |
14.1 |
18 |
27.5 |
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Total riparian width (width of the river channel plus woody vegetation) tends to diminish downstream, and it is clear that the largest portion of riparian area was lost in transects 3 through 6 after 1938. These transects are located in the portion of the valley that has experienced the greatest decline in the groundwater table. Changes in woody vegetation and channel area are far less dramatic in the other transects, as well as between 1978 and 2005.
Total woody vegetated area has decreased 42% from 1938 to 2005. In nine of the eighteen transects the maximum amount of woody vegetation was seen in 1938. However, 1938 also contained the minimum amount of woody vegetation in four transects. On average, woody riparian vegetation has not changed significantly between years (1938, 48%; 1978, 49%; 1989, 48%; 2005, 35%). These results indicate that changes in the riparian vegetation declined in proportion to changes in the river channel width.
The overall results from this analysis suggest that the observed narrowing of the river channel and proportional loss of riparian vegetation can be attributed to the combined effect of groundwater extraction and land conversion to agriculture. |
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