Streamwatch Project

Land Use

The exact cause(s) of degradation at various locations in White Clay Creek are not known, but often stream degradation can be linked to the human uses of water and land. Land use is variable in the watershed but mainly rural (agriculture) in Pennsylvania and mostly suburbanized (homes and industry) in Delaware. The watershed (269 km²) is a combination of forests (22%), agriculture (51%), and development (13%). Agriculture is a combination of cultivated crops (12%) and pasture/hay fields (39%). Development also varies from low intensity (e.g., single family homes) to highly urbanized (e.g., apartment complexes, row houses and commercial/industrial buildings) with more low (9%) and medium (3%) development than high (<1%). The remaining land is categorized as open developed space (e.g., grass lawns, gold courses; 9%), wetlands (3%), and barren land (e.g., rock, sand, clay; 1%). Another important land use variable is impervious surfaces (e.g., parking lots, roofs and road; 6%), which is a component of other land use categories (e.g., development).  Land use varies among sites with a range in the amount of forests (e.g., from 10% at Site 25 to 26% at Site 23), agriculture (e.g., from 49% at Site 25 to 81% at Site 19), and development (e.g., from <1% at Site 11 to 16% at Site 25). Population density can also be used to help describe the impact on the watershed. Densities (measured in 2000 census) among sites ranged from 33 people/km² at Site 11 to 468 people/km² at Site 24. Land use for each sampling location can be found on the individual site web page.

White Clay Creek and Land Use

Ordination diagrams (Non-Metric Multidimensional Scaling) on common macroinvertebrates densities that were averaged from 1997-2008. Sites (points) that are close together have more similar macroinvertebrate communities. Vectors (lines) indicate macroinvertebrate groups or land use variables that help explain patterns among sites.

All Sites

• Certain macroinvertebrates characterized good conditions (e.g., mayflies [Heptageniidae, Baetidae], caddisflies [Philopotamidae, Ueniodae, Glossosomatidae, Polycentropidae], and others [Psephenidae, Ceratopogonidae]). Sites on the Lower Mainstem lack these insect groups. Sites 18 and 16 had an abundance of round worms (Nematoda), a group considered tolerant to pollution.
• Good water quality (high MAIS rating) occurred in the most upstream or headwater sites (11, 19, 0) on the 3 main Branches.
• As the watershed becomes larger water conditions become more degraded (a decrease in MAIS score) corresponding to an increase in impervious surfaces (parking lots and roads), and developed areas (buildings). Although it is important to realize water quality is Poor (at Sites 6 and 16) before it ever reaches the Lower Mainstem (Site 14), so Poor conditions in the lower half of WCC should not be attributed solely to changes in land use.

Comparing the best sites in the headwaters (0, 11, 19) to the worst sites on the Lower Mainstem (20, 21, 22) there is:

• a 7 point decrease in the MAIS score (11 vs. 4)
• 3x higher population (76 vs. 231 people/km²)
• 5x more developed area (1.2 vs. 5.6% of the watershed)
• 4x more impervious surfaces (0.7 vs. 2.9%)

Upper Three Branches of White Clay Creek

To better understand what was occurring in the upper reaches (East, Middle and West Branches) a separate ordination was done. Similar to the analysis with all sites, a decrease in water quality was associated with an increase in developed and impervious surfaces in the watershed, as well as, an increase in human population. The best water quality (highest MAIS) was found in the headwaters (most upstream sites) and water quality degraded as it went downstream. Sites 18 (East) and 4 (Middle) are downstream from sewage treatment discharge but they have different macroinvertebrate communities suggesting the concentration of sewage effluent may vary or that there are additional factors influencing water quality at these sites.

For the three branches, comparing the best sites (0, 11, 19) to the worst sites (3, 18, 17) there is:

• a 5 point decrease in the MAIS score (11 vs. 6)
• 2x as many people (76 vs. 145 people/km2)
• 3x more developed area (1 vs. 3% of the watershed)
• 2x more impervious surfaces (0.7 vs. 1.5%)

Graphs show relationship between MAIS and two important land use variables (population density [people/km² in 2000] and % developed land) using sites in the upper three branches. There was a negative relationship between MAIS and population density (i.e., as the number of people increased the water quality decreased). Water quality also became more degraded as the % developed land increased.