Barrie Creeks, Lovers Creek, and Hewitt's Creek Subwatershed Plan

The Barrie Creeks, Lovers Creek and Hewitt’s Creek Subwatershed Plan
7.3 Agricultural interactions - land use, streams, and aquatic wildlife
When rain falls and flows over exposed soils on agricultural land, it can cause more erosion
than in natural areas, due to a relative lack of vegetation cover, and to picking up contaminants
not present in natural areas. Soil particles eroded by stormwater in agricultural areas often
have phosphorus adsorbed to them, particularly if the storm event happens relatively soon after
a surface application of fertilizer (Figure 7-2). As such, agricultural stormwater can contribute to
both the sediment loads and phosphorus loads in receiving water bodies. In fact, historically,
the conversion of much of the Lake Simcoe watershed to agricultural land in the mid 1800s
caused a spike in phosphorus loadings to the lake (Wilson and Ryan, 1988). Agriculture
remains a significant contributor of phosphorus to Lake Simcoe (Louis Berger Group, Inc.,
2010), and, as the largest land use in the Hewitt’s Creek subwatershed, is the largest
contributor of phosphorus to Hewitt’s Creek. Other contaminants, such as nitrates and metals
can also be washed off of agricultural lands and into nearby watercourses
The addition of contaminant-laden sediment to watercourses can have significant deleterious
impacts to aquatic ecosystems. Suspended sediment in watercourses increases the amount of
sunlight that is absorbed by the water, and thus can increase water temperature. At high levels,
it can also clog or abrade fish gills, impeding their ability to breathe, or cloud the water, reducing
the hunting efficiency of visual predators. As the sediment settles out of the water column, it
can blanket the substrate, covering important spawning habitat for species such as brook trout,
mottled sculpin, white sucker, and others. The addition of the phosphorus adsorbed to
sediment contributes to the eutrophication cycle, which is of great concern in the Lake Simcoe
watershed. Phosphorus acts as a fertilizer in aquatic ecosystems, causing increased growth of
aquatic plants, and most significantly in streams, algae. As the algae decompose, bacteria
involved in the decomposition process take dissolved oxygen from the water column. At high
levels of algae, this respiration can cause the amount of dissolved oxygen in watercourses to
decline to critical levels, making them less suitable as habitat for fish and other aquatic
organisms (Figure 7-2).
An issue which is specific to the management of agricultural watersheds are agricultural drains.
These drains include both open ditches and tile drains which typically are installed in areas with
poor natural drainage, to improve agricultural productivity. Ditches, or open drains, are typically
straightened to quickly remove water from the area and have limited riparian vegetation. To
ensure they continue to work properly, they require maintenance, which can lead to the
alteration or removal of remaining vegetation, and disruption and change to the substrate. In
addition, their intended function of rapidly draining wet soil has the unintended consequences of
changing the rate and timing of peak flows, and increasing the volume of phosphorus and
sediment travelling from agricultural fields to Lake Simcoe. As an example, the conversion of
much of the headwaters of Hewitt’s Creek from natural watercourses to agricultural drains, and
the resulting loss in water storage in the summer, would help to explain why aquatic
communities are degraded in these watercourses (Figure 5-3, Figure 5-6, Figure 5-12). In cases
where these drains bisect wetlands, they can cause the water table to drop, decreasing the
extent and hydroperiod of ephemeral wetland pools, which can lead to a loss of breeding habitat
for frogs and salamanders, and migratory habitat for waterfowl (Figure 7-2).
Another issue particular to agricultural lands is the degradation of water quality and riparian
areas where livestock have access to watercourses. The input of urine and manure directly into
the water and onto low lying nearby fields, where it can be washed into the watercourse, affects
water quality. The livestock can also trample streambanks, which contributes to instability and
erosion, and sedimentation in the stream; while livestock in the stream can destroy spawning
habitat (Figure 7-2).
Chapter 7: Integration and implementation 318