2.2.4 Soils

Following the glacial ice's retreat, physical, chemical and biological processes turned the upper 2 to 4 feet of drift material into the soil layer that today covers the watershed.  This relatively thin soil mantle largely controls how people may use the land. 

Because traits of the soil directly influence runoff, they affect total water volumes generated in the watershed.  Urbanizing undeveloped drainage basins can subsequently increase stormwater runoff rates and volumes.  To estimate and help manage this runoff, the Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service) has indexed over 4,000 soil systems into four major hydrologic soil groups.  This classification relies on two major processes: infiltration rate and transmission rate.  Table 7 lists the four major hydrologic soil groups defined by the NRCS, and Figure 5 illustrates their distribution across the watershed.

Table 7.  Natural Resources Conservation Service soil hydrologic group interpretation.

Hydrologic Group

Depth to Water Table (ft)

Dominant Slope (%)

Hazard of Sheet Erosion

A

>6

0-6

Slight

B

>2

2-18

Moderate

C

>2

0-2

Slight

D

<2

0-2

Slight

 

Soil Group

Description

A

Soils with low runoff potential. These soils have a high rate of infiltration even when thoroughly wet.  They are deep, well to excessively drained sands or gravels.  These soils also have a high rate of water transmission.

B

Soils with a moderate infiltration and transmission rate when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well- to well-drained soils with moderately fine to moderately coarse textures.  Water movement through these soils is moderately rapid.

C

Soils with a slow rate of infiltration rate when thoroughly wet. Water movement through these soils is moderate or moderately slow and they generally have a restrictive layer that impedes the downward movement of water.

D

Group D consists of soils with a high runoff potential (soils having very slow infiltration rates).  These soils have a slow infiltration rate because of clay content, high water table, or claypan or clay layer at or near the surface.  These soils have a very slow rate of water transmission.

A/D

B/D

C/D

Dual hydrologic soil groups are given for certain wet soils that could be adequately drained. The first letter applies to the drained and the second to the undrained condition. Soils are assigned to dual groups if the depth to a permanent water table is the sole criteria for assigning a soil to hydrologic group D.

Source:  Natural Resources Conservation Service.

Surface conditions control the infiltration rate, or the rate at which water enters the soil.  How fast water moves through the soil, the transmission rate, depends upon soil structure and composition.  Soil type, plant cover, surface retention and the percentage of impervious surfaces also affect runoff volumes.  The District's 2003 Hydraulic, Hydrologic, and Pollutant Loading Study (HHPLS) used soil and other features to assess infiltration potential within the watershed.  Each subwatershed plan includes a figure detailing infiltration potential within the subwatershed; Figure 6 shows infiltration potential across the entire watershed.  The upper watershed has generally a medium potential for infiltration, with significant areas of variable or low potential associated with organic soils.  The lower subwatershed is largely of medium to high potential, due mainly to the surficial till deposits that overlay the bedrock in that area.