Lecture 10 - Introduction to Stream Ecology and Physical Environments

Overview of the Stream Ecology Block

• Sequence of Lectures:     * The current block consists of eight lectures focused specifically on stream ecology.     * The stream ecology block will be followed by lectures on invasion ecology conducted by Ian Dagon.     * The course will conclude with applied issues regarding biomonitoring and restoration.

• Defining Feature of Fluvial Systems:     * The primary characteristic that differentiates streams and rivers from lake ecosystems is the movement of water.     * The physical setting of the stream acts as the "template" for all subsequent ecological processes and biological adaptations.

Climate Change and Recent Disturbance Events

• Current Climate Context:     * Climate change is not a future threat but a present reality, evidenced by recent flooding in Wellington.     * Warming temperatures lead to increased water vapor in the atmosphere, resulting in more frequent, intense, and severe precipitation events.

• Case Study: Mount Pironga and Rangatukemia Stream:     * The Pironga District (Waipa area) experienced intense rainfall in February, causing large-scale flooding.     * Specific Impacts at Rangatukemia Stream:         * A road culvert was overtopped by the stream, creating a massive temporary lake upstream and causing significant downstream damage.         * Post-flood consequences include heavy mud deposition and the death of riparian vegetation.         * Disturbed State: The channel remains in a state of flux where the stream spilled out to create a new secondary channel.         * Physical Observations: Previously stony bottoms and deep pools (specifically those inhabited by large eels) have been covered by a layer of fine, gray sediment.

• Field Trip Objective:     * Students will collect benthic samples for macroinvertebrates at the Rangatukemia Stream (Hamilton class) and Utitatta Stream (Tauranga class).     * Data will be compared with pre-flood data from the previous year to analyze the effects of disturbance.

The Four Dimensions of Rivers and Streams

• Ward’s Conceptual Framework (Jim Ward):     1. Longitudinal Dimension: From upstream to downstream (source to sea; Ki uta ki tai).     2. Lateral Dimension: The connection between the river and its floodplains.     3. Vertical Dimension: The interface between surface water and groundwater (the hyporheic zone).     4. Temporal Dimension (Time): The dynamic nature of these systems through time, particularly how lateral connectivity fluctuates with flow.

Hydrologic Cycle and Groundwater Dynamics

• Variation in Hydrology: Differences in climate and topography (e.g., desert streams vs. West Coast New Zealand streams) dictate the ecology of the system.

• Soil Permeation: Water infiltrates the soil through root channels, animal burrows, and soil pores until it reaches an impermeable layer like bedrock.

• Water Zones:     * Saturated Zone: The water table.     * Vadose Zone: The unsaturated area above the water table where water is held by capillary action on soil particles.

• Infiltration and Land Use:     * Pastoral Systems: Livestock cause soil compaction, reducing infiltration rates and increasing overland flow.     * Urban Systems: High levels of impermeable surfaces (roads, roofs, car parks) lead to "flashy" hydrographs, where water is rapidly channeled into streams.

• Hydrographs: A graphical representation of discharge ($Q$) over time.     * Rising Limb: The dramatic increase in discharge following rain.     * Recession Limb: The attenuation and fall of flow post-precipitation.

Drainage Networks and Stream Ordering

• Network Patterns:     * Dendritic: Tree-like branching (e.g., Waikato River, Clutha-Matau).     * Parallel: Streams cutting through soils like those in Te Puke/Paengaroa.     * Radial: Drainage flowing outward from a central peak (e.g., Taranaki Maunga).

• Stream Order Measure:     * First-Order Stream: Has no tributaries; located at the headwater source.     * Second-Order Stream: Formed by the convergence of two first-order streams.     * Third-Order Stream: Formed by the convergence of two second-order streams.     * Statistics: $80\%$ of the world's streams are of the first to third order.     * The maximum order is the $12^{th}$ order, represented by the Amazon River.

Habitat Hierarchies and Catchment Zones

• Frissell Scale of Hierarchical Organization:     * Stream System (Small Catchment).     * Segment System ($100s$ of meters).     * Reach System (Approx. $100\,\text{m}$ or less).     * Microhabitat.

• Catchment Scaling: The length of a river scales predictably with the catchment area. For the Waikato River, calculations predict a length of approximately $438\,\text{km}$.

• Three Major Geomorphic Zones:     1. Source Zone (Erosion): High elevation, steep gradients, high stream power.     2. Transition Zone (Transport): Sediment is mobilized and moved through the system.     3. Floodplain Zone (Deposition/Response): Low gradient, fine materials settle out, increased discharge, width, and depth.

Channel Morphology and Physical Features

• Channel Forms: Straight, Meandering, Braided, or Anabranching (forming islands).     * Example: The Upper Rakaia River is a prominent braided river in New Zealand.

• The Pool-Riffle System:     * Riffle: Shallow, fast-flowing, erosional area with broken water and larger substrate.     * Pool: Deeper, slow-moving, depositional area.     * Sinuosity: Created by point bars constricting flow.

• Large Wood (Woody Debris):     * Increases habitat heterogeneity.     * Increases the number of pools (step-cascade systems).     * Used in restoration to create microhabitats for fish.

Hydrogeomorphic Processes

• Discharge Calculation: $Q = ext{Width} \times ext{Depth} \times ext{Velocity}$.

• Hjulström Curves: These curves define the critical flow velocities required to either erode (mobilize) or deposit substrate based on particle size.     * Increased flow velocity in floods changes the critical threshold for particle movement.

• Flow Profiles:     * Vertical: Fastest flow is at the water surface; slowest at the bed due to friction.     * Horizontal: In a bend, the fastest flow is at the outer "cut bank," while the slowest is at the inner "point bar."     * Mean Velocity Measurement: Standardized at $0.6 \times \text{depth}$.

• Stream Power: A function of discharge and channel gradient ($\text{Power} \propto Q \times \text{gradient}$).

• Froude Number ($Fr$): A dimensionless velocity-depth ratio used to describe meso-habitat conditions (e.g., tranquil pool vs. broken riffle).

• Bed Roughness: Created by boulders and cobbles; provides low-velocity refugia for fish and macroinvertebrates against the current. Quantified using Manning’s equation.

The Impact of Floods on Stream Ecology

• Poetic Description: "Streams are the gutters down which flow the ruins of continents" (associated with Luna/Aldo Leopold).

• Physical Hazards during Floods:     * Bed movement: Rocks tumbling, crushing organisms.     * Increased suspended sediment: Causes abrasion and scouring (comparable to a underwater sandstorm).     * Scouring: Removal of biofilms and periphyton (food sources).     * Loss of riparian vegetation via bank erosion.

• Natural Resilience and Strategies:     * Fish: Move out of the main channel into stable areas.     * Macroinvertebrates: Burrow into the stream bed or utilize aerial dispersal (adult insects flying away from the disturbance).

Questions & Discussion

• Question: Does the upcoming online test cover this week's material or just weeks 1 through 5?

• Response: The test will not cover anything from today's lecture. It covers only the Plankton and Lake Ecology lectures provided by Ian and Dennis.

Logistics and Administrative Reminders

• Field Trip Schedule:     * Hamilton Students (Saturday): Meet at Gate 9 at $8:15\,\text{AM}$; departure at $8:30\,\text{AM}$. Site: Rangatukemia Stream on Mount Pironga.     * Tauranga Students (Sunday): Meet at Durham Street Lane at $8:45\,\text{AM}$; departure at $9:00\,\text{AM}$. Site: Utitatta Stream near Katikati.     * Requirements: Health and Safety forms, sensible clothing for chilly mornings, water, and lunch (no nearby shops).

• Online Test 1:     * Opening/Closing: Opened last night; closes tomorrow at $11:30\,\text{PM}$.     * Format: $22$ multiple-choice and short-answer questions.     * Rules: One attempt only; strictly $1\,\text{hour}$ to complete; open book; check for generative AI use (results in zero marks).